You've got the shivers, the fever and the body aches. It must be the flu. Or is it your period?

It's true. Some women experience flu-like symptoms before and during their period. The period flu, as some people refer to it, includes a combination of symptoms categorized under PMS that last no longer than two weeks.

"Period flu is a cluster of physical, emotional and psychological changes that occur prior to the menstrual cycle," said Nichole Butler-Mooyoung, M.D., a gynecologist at Weiss Memorial Hospital Women's Health Center. If symptoms are very severe, this can be a case of premenstrual dysphoric disorder (PMDD).

Doctors aren't entirely sure why these symptoms occur, but they theorize it may have something to do with the body's response to changing hormone levels.

So, what causes period flu?

Before a woman begins her period, progesterone will rise and peak to prepare for menstruation. During this phase (called luteal), a woman's immune system will decrease and continue to drop throughout her period