My daughter had severe pain that gradually got worse over a few years. They put her thru a couple years worth of tests and still didn't know what was wrong. She was 28 and had 4 kids so they offered her a hysterectomy. AFTER they removed her uterus they were able to diagnose and it was enlarged uterus. Basically what it is, not all of the lining sheds each month and it builds and causes circulatory congestion in the area, pain all month and severe pain with periods. Maybe you could be checked for it, but they may not offer hysterectomy since you have no kids. (It did not interefere with her having kids.)

I hope you find relief soon.