My fiancee also like that. She went to the gynae for a check. They say it is due to the wall thickening or something like that. To alleviate of the pain, she will need to go through a surgery to remove the flesh of her uterus or sth like that. However it isn't permanent and it will grow back after a few years. The only way to permanent rid of this issue is to get pregnant.

For now, she's using painkillers to alleviate the pain.