Pain In The Ovary During Intercourse. Do Not Have STD. Other Reasons?

Hello, for the past 3-4 years, I have had mild pain during sex. From about 2 weeks before my menstrual cycle and all the way through it. But over the past 1 1/2-2 yrs. the pain will be so severe that I wouldn t wish on my worst enemy. It feels like a menstrual cramp , but 1000x worse, and secluded to one ovary at a time. Rarely both at the same time. but both a seperate times. Been tested for STDs, so it s not that. Help! Also, I m in my early 20.