Two things are happening that you cannot see. Your body is holding on to more water than usual, and your gut has slowed right down (so everything is moving through you more slowly and taking up more room on the way).

In the second half of your cycle, progesterone rises. Progesterone relaxes smooth muscle (including the muscle that moves food through your gut) so digestion slows and gas hangs around longer. At the same time, shifting oestrogen and progesterone change how your kidneys handle salt and water, so you retain more fluid.

It usually peaks in the day or two before your period and settles once bleeding starts.

• Own a pair of jeans that fits your bloated days, not just your flattest ones. This is a wardrobe problem, not a body problem.
• Drinking more water genuinely helps (dehydration makes your body hold on tighter.
• Walking helps things move. So does not weighing yourself that week.
• Log it in your Cycle Studio. Once you can see it turning up at the same point every month, it stops being a nasty surprise.

Colour is not telling you about the blood. It is telling you about the speed. Brown blood is simply blood that took longer to come out.

Blood contains iron. When blood is exposed to oxygen for longer, that iron oxidises (the same process that turns a cut apple brown or makes metal rust) and it changes from bright red to dark red to brown.

At the very start and the very end of your period, flow is slow, so blood spends longer in your uterus and vagina before it leaves. Slow exit, more oxygen, browner colour. In the middle, when flow is fastest, it comes out fresh and red.

• Stop worrying about brown days at the start and end. That's just the pace changing.
• Brown spotting at a completely random point in your cycle, month after month, is worth mentioning to a nurse or GP (that one is different).
• Same for discharge that smells strongly bad alongside it.

Your uterus is a muscle, and during your period it is squeezing (hard, and repeatedly) to push the lining out. You cannot feel it working, only the ache it leaves behind.

Chemicals called prostaglandins make the uterus contract. The more prostaglandins you produce, the stronger the squeezing, and strong contractions briefly reduce the blood supply to the muscle, which is what turns 'tightening' into 'pain'.

The nerves that serve your uterus share pathways with the nerves from your lower back and upper thighs. Your brain can't always tell exactly where the signal started, so it reports pain in all of them. That's called referred pain, and it's why period pain rarely stays where you'd expect.

• Heat genuinely works (a heat pack on your lower back or tummy relaxes the muscle and eases the squeezing.
• Anti-inflammatory painkillers like ibuprofen work on prostaglandins specifically, which is why they often help more than paracetamol. Check with a parent or pharmacist about what's right for you.
• Taking them when the pain starts, rather than waiting until it's unbearable, works better.
• Gentle movement helps more than lying completely still, even though lying completely still is more appealing.

Your body is not running at the same cost every day of the month. In the second half of your cycle it is quietly burning more energy at rest, before you have done anything at all.

After ovulation, your resting metabolic rate rises (for many people by somewhere around 510%. Your core body temperature goes up slightly too. Both cost energy, so genuine hunger increases.

On top of that, the hormone changes in that phase affect serotonin, which is partly why the craving is so often specifically for carbohydrates and sugar rather than just "more food".

• Eat the bigger lunch. Fighting it usually ends in feeling worse and eating more later anyway.
• Pack an actual snack for that week rather than trying to run on nothing until 4pm.
• Notice which days it lands on (in the Cycle Studio it usually shows up in the same window every cycle.
• If eating starts to feel scary, shameful, or like something you have to control all the time, that's worth telling an adult you trust. That's a really common thing to need help with.

It's not a mess your body is making. It's a cleaning system running constantly, without being asked, keeping everything healthy and getting rid of what shouldn't be there.

Your cervix produces fluid that carries away old cells and bacteria, and keeps the whole area at the right acidity so unfriendly bacteria struggle to grow. The amount and texture change through your cycle as oestrogen rises and falls (drier after your period, stretchy and slippery around ovulation, thicker and creamier afterwards).

Having none at all is more likely to be a problem than having plenty. And it never, ever needs soap. Washes and scented products push it out of balance and usually make things worse.

• Wash the outside with plain water. That's it. That's the routine.
• Cotton knickers, and a liner on heavier days if you'd rather.
• Log it in the Cycle Studio (the stretchy days are a decent clue that you're around ovulation).
• Green, grey or dark yellow, a strong bad smell, or itching and soreness are your body flagging something. That's a nurse or GP, not more washing.

In endometriosis, tissue similar to the lining of the uterus is growing in places outside it. Around the ovaries, the tubes, elsewhere in the pelvis. And it responds to your hormones every month, exactly like the lining inside does.

That misplaced tissue thickens and breaks down with your cycle, but the blood and tissue have nowhere to go. That causes inflammation, irritation and, over time, scar tissue. Which is why the pain can be severe, and why it doesn't always stay confined to your period days.

Doctors don't yet agree on one single cause. What is well documented is that it takes far too long to diagnose, on average around eight years in the UK (largely because severe period pain gets normalised).

• Track it. Dates, pain scores, what you had to cancel. A GP takes a pattern far more seriously than a memory.
• Say the specific sentence: "this pain is stopping me going to school." That lands differently to "my periods are bad."
• You're allowed to ask for a second opinion, and to bring someone with you.
• Endometriosis UK has good, plain-English guidance if you want to read more before you go.

There's no shared scale, so almost everyone assumes theirs is average. The measure doctors actually use isn't a volume, it's the impact on your life.

Signs that count as heavy: soaking through a pad or tampon every hour or two for several hours, needing to double up, flooding through clothes or bedding, clots bigger than a 10p coin, or bleeding lasting more than 8 days.

It matters beyond inconvenience. Losing a lot of blood month after month can leave you low on iron, and low iron is why heavy periods often come with exhaustion, dizziness, breathlessness and looking pale. That part is very treatable once someone actually checks.

• Track two things for two cycles: how often you change protection, and what you couldn't do.
• Ask specifically whether your iron levels should be checked.
• Take someone with you if adults have brushed it off before. It shouldn't be necessary, and it often helps.
• "Everyone gets cramps" is not a medical assessment. You're allowed to say so, politely.

You are not one steady version of yourself with random bad weeks. The second half of your cycle runs on a genuinely different hormone mix to the first half (different fuel, different temperature, different mood chemistry).

The luteal phase is the stretch between ovulation and your next period (usually around 12 to 14 days). Progesterone rises through it, which raises your core temperature slightly, slows digestion, and has a sedating effect for some people.

Then, if there's no pregnancy, both progesterone and oestrogen fall away fairly sharply in the last few days. That drop is what triggers your period, and it's the drop, more than the levels themselves, that most affects mood and sleep.

• Put the demanding stuff (the presentation, the big social thing (in the first half of your cycle where you can choose.
• Plan your second half around lower energy instead of being annoyed by it.
• Protect your sleep in the last few days. Everything else gets harder when that goes.
• The Cycle Studio will show you your own version of this. Yours won't be textbook, it'll be yours.

One ovary releases an egg. It takes minutes. You most likely won't notice it happening (but you may well notice the build-up to it, because oestrogen peaks just before)

Oestrogen climbs through the first half of your cycle, then triggers a surge of luteinising hormone, which releases the egg. Around that point some people notice a one-sided twinge low down, clearer and stretchier discharge, and more energy or sociability.

The second half of the cycle is fairly fixed in length. The first half is the bit that varies - so if your cycle is 24 days or 34 days, ovulation moves accordingly. It is genuinely not day 14 for most people.

• Notice the stretchy, slippery discharge (it's the most reliable free signal you've got.
• If you get a one-sided twinge mid-month, that's likely mittelschmerz, and it's harmless.
• Use those days for the things that need you at full volume, if you get the choice.
• Cycles that are still settling may not ovulate every month, especially in the first couple of years. That's expected, not broken.

They are related, they're all downstream of the same hormones. When the balance shifts, several apparently unconnected things shift together.

In PCOS, levels of androgens (hormones everyone has, in differing amounts) run higher than usual. That can mean ovulation happens irregularly or not every cycle, which is why periods become unpredictable or disappear. Higher androgens also drive oilier skin, acne, and hair growth in a more typically male pattern.

Many people with PCOS also have insulin resistance, meaning their body has to work harder to manage blood sugar( part of why weight can be harder to shift despite doing nothing differently).

• Track your cycle lengths. "My periods are irregular" is vague; "my last four cycles were 45, 62, 38 and 51 days" gets you taken seriously.
• PCOS is diagnosed on a combination of things, not one test, and it can be harder to diagnose in teenagers because irregular cycles are normal early on.
• Ask about it directly if the pattern sounds like yours. You're allowed to bring it up first.
• Verity is the UK PCOS charity, if you want to read more.

Concentration isn't a fixed setting. It's the output of sleep, pain, mood and hormones, and in the days before your period, several of those quietly get worse at once.

Being straight with you: research on whether hormones directly measurably change how well you think is genuinely mixed, and the effects found tend to be small. So we're not going to tell you your IQ drops.

What isn't in doubt is that poor sleep, pain and low mood all measurably reduce working memory and concentration, and all three are more likely in the late luteal phase. The fog is real. It's mostly arriving via those, rather than hormones scrambling your brain directly.

• Write things down that week instead of trusting your memory. This is not defeat, it's admin.
• Treat the sleep and the pain (that's usually where the fog is actually coming from).
• Don't schedule the thing that needs deep focus for that window if you have any say in it.
• Don't make big decisions about your own capability on your worst-sleeping days.

The chemical making your uterus contract doesn't stay politely inside your uterus. It leaks into nearby tissue, and your bowel is right there.

Prostaglandins (the chemicals that make the uterus contract to shed its lining) also affect the smooth muscle of the intestines. More prostaglandins means more bowel contractions, so things move through faster and looser. That's the "period poops".

The flip side explains the other half of the month: in the luteal phase, higher progesterone slows the gut down, which is why constipation before your period and the opposite once it starts is such a common pattern.

• Water and fibre in the days before help with the constipation half.
• Anti-inflammatory painkillers reduce prostaglandins, so they often calm both the cramps and the bathroom situation at once.
• Know it's coming on day one and plan the day accordingly. That alone removes most of the panic.
• It is astonishingly common. Nobody talks about it, which is exactly why everyone thinks it's just them.

Your hormone levels are usually completely normal. The difference is in how sensitively your brain responds to them changing.

PMDD is a recognised condition affecting a small percentage of people who menstruate. It's understood to involve an unusual sensitivity to normal fluctuations in progesterone and its by-products, which affect the brain's GABA and serotonin systems (the ones handling mood, anxiety and calm).

The pattern is the giveaway: symptoms appear in the week or two before a period, and clear within a few days of it starting. Every cycle. That timing is what separates it from other mental health conditions, and it's what a doctor will look for.

• Track symptoms against your cycle for two or three months. That chart is the single most useful thing you can hand a GP.
• Say the timing out loud: "this happens every month before my period and stops when it starts."
• Tell someone you trust which week it is, before it arrives, so you're not managing it alone.

Nothing about the situation changed. What changed is the chemistry you're processing it with (the same event genuinely lands harder depending on where you are in your cycle).

After ovulation, oestrogen and progesterone rise, then fall away in the last few days before your period. That fall affects serotonin (one of the chemicals involved in mood, sleep and appetite) which is why irritability, tearfulness, cravings and poor sleep so often turn up together, as a set.

The physical symptoms come from the same shift: fluid retention makes breasts feel sore and heavy, and slowed digestion causes bloating.

• Know which days they are. Context doesn't remove the feeling, but it stops you believing everything it tells you.
• Give yourself an extra 10% that week and take one thing off the list.
• Protect sleep (it's usually the first domino, and everything else gets worse when it goes.
• If it's severe enough to wreck your friendships, schoolwork or safety every single month, read the PMDD entry. That's a different thing and it deserves a doctor.

Your body temperature isn't the same all month. In the second half of your cycle it runs slightly warmer (and falling asleep depends on your body being able to cool down.

After ovulation, progesterone raises your core body temperature by roughly 0.30.5°C. Dropping your core temperature is part of how your body triggers and maintains sleep, so a warmer baseline tends to mean lighter, more broken sleep.

Then progesterone falls sharply in the last days before your period (and because it has a calming effect on the brain, losing it quickly can bring restlessness and 3am wake-ups. Add cramps and you have a genuinely bad week for sleep.

• Cooler room, lighter duvet, fewer layers for that week specifically.
• Guard your bedtime harder than usual in the few days before (this is the domino that knocks over mood, concentration and hunger.
• Log it. The Cycle Studio will spot it as your earliest clue if it's yours.
• Weeks of broken sleep with no cycle pattern is a different thing, and worth mentioning to a GP.

Two separate things are happening at the same time, and they get mistaken for one. Your body genuinely is slightly different (and, separately, your tolerance for how you look has dropped.

The physical half is real: fluid retention makes your face puffier and your stomach fuller, and hormone shifts can bring breakouts. So your body has actually changed a bit.

The other half is that studies consistently find body dissatisfaction fluctuates across the menstrual cycle, tending to peak in the premenstrual days. Falling oestrogen and serotonin affect mood and self-criticism generally (and "how I look" is simply one of the things a low, self-critical mood attaches itself to.

• Check the date before you believe the thought. If it's the same three days as last month, treat it like the sore boobs (a symptom, not a revelation.
• Don't make decisions about your body on those days. No new diets, no cutting your own fringe, no deleting every photo.
• Wear the clothes that fit today's body rather than punishing yourself with the ones that fit last week's.
• If the feeling doesn't lift when your period starts, or it's there most of the month, that's worth telling someone about. That's a different thing to a cycle dip.