It could be a couple of things. My daughter at 18 had this and was diagnosed with polycystic ovarian disease. You should speak to your doctor about it.
When I turned 36, I started having the problems you describe (in fact, it was so bad I missed a couple of days of work each month). They were talking hysterectomy but then decided to give me a D&C (I had heavily built up uterine lining) and put me on Depo Provera shots to control it.