This is one of those things that you can't really talk about with anyone, but just about every menstruating woman who also loves to work out understands on a visceral level: that intense terror you feel when you can tell your tampon is about to fail mid-workout.
The good news is, if this has happened to you, you're not alone. "It's completely normal. It's all mechanics. Squatting is one of the exercises that can help tighten your vaginal muscles over time. But if you have a tampon in, it might leak because your muscles down there are squeezing it. May even get pushed out of the vagina entirely," says Draion Burch, M.D., an ob-gyn based in Pittsburgh, PA (a.k.a., Dr. Drai). You may notice this happens when you go into bridge pose too, since that also tenses your vaginal muscles, he adds.
If it happens to you every time you're at the gym, you could try using a more absorbent tampon (like "super" instead of "regular"), and make sure to change to a new one right before working out, Dr. Drai suggests. Or switch to a menstrual cup, which is less likely to get pushed out by muscle contractions.
You could also try wearing "period panties" like Thinx (from $24, shethinx.com) with a tampon (or without one, depending on how heavy your flow is) for extra insurance; they're super-absorbent and will catch any workout leaks.
Dr. Drai also says you could make your heavy flow days your rest days. Not always practical? Then learn about six ways to stop your menstrual cycle from ruining your workouts instead.