I agree with Rusty. My sister was having two periods in one month, so she went to the doctor and they discovered a polyp on her ovary. She got the polyp removed, but the problem was still occurring. Then, she got this wild hair to ask her dermatologist if abnormal bleeding could be a rare side effect of the medication that she was taking, and it was! So, she stopped taking the medicine and her period became regular again. So, basically, there are many things that could cause abnormal bleeding. Just be sure to check with ALL your doctors!