We all get headaches from time to time—some so debilitating that the best we can do is retreat someplace dark and quiet and hope that it passes quickly. But if you work out regularly, you may be surprised at the real reason your head is feeling hammered.

We asked headache experts to share some of the most common causes of head pain among active women. Click through to check them out.

1. Your time of the month

Women are three times as likely as men to develop migraines, in large part because our hormones are often all over the place. Some women may develop what’s called menstrual- related migraines, triggered by a drop or a rise in estrogen levels, typically about two days before you get your period and/or in the  rst three days of your cycle. According to the American Headache Society, these headaches can be especially challenging to treat because they don’t always respond to the same medications that might work at other times.

2. Dehydration

Not getting enough water is a common reason your head may start to hurt. “When you’re dehydrated, your brain doesn’t get enough blood volume, so it may send out pain signals in response to this lack of fluids,” says Natbony. Some of us are also more susceptible to dehydration headaches, especially if you have a history of migraines. Aim to drink about 64 ounces of water daily or more if you have a heavy workout scheduled.

3. You skipped breakfast

Too crazy-busy to sit down and eat? Don’t be surprised if a headache comes on, warns Pamela Peeke, M.D., author of Body for Life for Women and a spokeswoman for the American College of Sports Medicine. “It’s very possible to develop a blistering headache due to low blood sugar,” says Peeke. That’s especially true if you’ve planned a tough workout while you’re fasting or are just underfueled. “If you’re lifting heavy or doing a hard HIIT class, think to yourself, ‘When was the last time I ate?’ If you’re not sure, it’s a good time to have a healthy snack.” Ward off a headache by keeping a small bag of nuts, an energy bar, or some other quick fuel in your gym bag, she adds.

4. A tough day at work

Whether you screwed up a presentation, got into a spat with your spouse, or found yourself stuck in a monster traffic jam, stress is often a trigger for a tension headache. “The muscles along the back of your neck connect to your upper back as well as the cap of muscles at the top of your head,” says Peeke. “When you’re stressed, you tighten up these muscles, which can create major head pain.” To release some of this tension, try doing some gentle stretching, like shoulder rolls and cross-body arm stretches, or take a yoga class to relieve tension all over.

5. A hard HIIT routine

Some women tend to develop what’s called a primary exercise headache. These are often severe with both sides of the head throbbing, tend to occur during or right after any form of strenuous exercise, and can last anywhere from a few minutes to a couple of days. If you notice a pattern of these headaches, see your doctor. She may prescribe medications that you take prior to exercise or a daily treatment. Changing your routine, toning down the intensity, and gradually ramping up your workout can also help. The good news: This headache pattern usually only lasts a few months.

6. A heavy upper-body day

Tension headaches can also be caused by holding too much muscle tension in your neck and shoulders—so doing a lot of back or shoulder work can be trouble if your form is off or you’re too tight, says Natbony. Take a few minutes to warm up before getting into bigger lifts, and try to relax and breathe throughout your training.