Getting your period usually isn't just a matter of bleeding out of your vagina; it often comes with a bonus pack of other irksome symptoms that change the way your body feels and how you act. You might feel puffy or bloated, or even like you just gained a few extra pounds, but this is just a hormone thing and effectively water weight, says Joshua Holden, MD, Assistant Clinical Professor of General Obstetrics & Gynecology at Columbia University Medical Center. In other words, don't worry about it.
Right before someone's cycle, estrogen and progesterone levels drop, Dr. Holden says. Then, after that person ovulates (when the egg is released), the estrogen and progesterone levels both boost for a week, then sharply decrease for the week until your period begins, he says. Progesterone somehow makes you retain water and gas (it's not totally clear why), which make you feel bloated, he says. "These symptoms are associated with feeling heavier but may also be implicated in actual weight changes," Dr. Holden says.
Exactly how much water you retain and "weight" you gain varies from person to person, and Dr. Holden says there's not really a normal or abnormal amount; if you were to step on a scale, you might not even see the fluctuations. For some people, PMS symptoms might be hereditary. It can feel a little uncomfortable if your clothes are tighter, but this "weight gain" (or any for that matter) really isn't something you need to worry about. Not to mention, it typically goes away after your period.
If you think you're bloating a lot more than usual, or it's not cyclical with your period, keep track of your symptoms and see a doctor — it could be a sign of other GI issues like IBS. While your hormonal changes are inevitable, there are a few things you can do to feel less puffy: Eat less salt, exercise a little, and drink water. Periods can suck, but it'll be over before you know it.