This seems like a fairly serious problem. I suggest that you first see an OB-GYN and then a specialist, if necessary. Fainting during your menstrual cycle could be serious. Your OB-GYN should ask several questions and perform a thorough physical exam. including a pelvic exam, a Pap smear, blood tests (to check hormone levels), or urine tests. If the issue is caused by structural problem(s) or some sort of growth, an ultrasound or CAT scan may be performed. Together, these tests can help the doctor determine how the fainting issue should be handled.

To track what to do in other cases of abnormal menstruation, check out this menstrual flow chart from the American Academy of Family Doctors. Good luck and update soonest.

I would think it could be a number of things. I agree with rooted, you should see your GYN first and take it from there.
It could be something like anemia which they will check for. I had a similar problem, I would get VERY lightheaded, never passed out, but this would happen during my period and it turned out my iron was really low. I got on supplements and I am back to normal.
good luck.

Hello everyone I finally went to my PCP had a battery of test done, all blood work normal, eeg normal, tilt table abnormal. I was diagnosed with High Blood pressure years ago however, I now have orthostatic hypotension, this causes me to get light headed and or pass out after I've been in a stooped postion it gets worse during my menses. My BP meds were lowered and I now have to be careful when standing, take it slow and easy and not jump up real fast. I'm blessed that it was nothing more serious and can be handled with such ease. Thanks for your comments.