One word: prostaglandins. Pesky prostaglandins are released even before your period starts to do the work of dislodging the uterine lining from the wall of the uterus so you can have period flow. If they only worked locally on the uterus, you would just have some cramps, but these actors find their way into the blood stream and to nearby structures like the intestines -- and can wreck havoc! Prostaglandins can cause intestinal cramps, diarrhea, nausea, vomiting, a feeling of being flushed, and general achiness. In short, they are no fun! To combat them, taking ibuprofen three times a day for the few days prior to your period can block the effects of prostaglandins and stop the side effects in their tracks. You’ll still get your period as expected but will have many fewer cramps and other unpleasant flu-like side effects.
If you mean you have flu-like muscle aches and fatigue before your period, then I say welcome to the club! A lot of girls and women (me included) have all-over aches pre-period. It's a relatively common PMS symptom. Some girls also experience nausea or even vomiting during PMS. You can try to combat PMS symptoms by drinking plenty of water and taking over-the-counter ibuprofen (according to package directions). Many PMS symptoms are caused by an excess of prostaglandins in our system, and ibuprofen can help balance that.
What exactly are flu-like symptoms? I should probably know this because whenever I’m suffering from something worse than a cold, I’m quick to say I have the flu. WebMD says flu symptoms include a high fever, headaches, muscle aches, chills, tiredness, maybe a cough and/or runny nose, and some stomach symptoms. Ah, that’s enlightening. Thing is, some of those symptoms are similar to PMS (at least, similar to when I get PMS). The couple of days before my period, I get some headaches, slight muscle aches, and I find that I’m a little more tired than usual. Let’s also see what Dr. Molly has to say about the matter.