6 Unexpected Symptoms Women Shouldn't Ignore

Best-case scenario: You've got an inner-ear issue or sinus congestion, or you just pushed it a little too hard with today's HIIT session.

Why it could be more serious: If you're drinking water, working out in comfortable temperatures (i.e., not sky-high heat and humidity) and it's an activity that's never caused you problems before, you could be looking at a heart issue. Two possible culprits: an irregular heartbeat or a problem with the valves that keep blood flowing through your heart in the right direction. "You heart isn't able to pump enough blood to the rest of your body, which is why you feel lightheaded," says Leslie Cho, MD, head of women's cardiology at the Cleveland Clinic. Irregular heartbeats are fairly common and most are actually harmless, but some, like atrial fibrillation, can increase your risk of stroke and heart failure. Same goes for heart valve disease—some forms don't cause major problems, but more serious ones can make stroke, heart failure and blood clots more likely. So if you're experiencing lightheadedness with no obvious cause, see your internist.