Cramping but not pregnant?

For the past few months I have been having uterus pain... It doesn't feel like period cramps but almost like my uterus is stretching. I've been on birth control for 2 1/2 years, so I'm not ovulating (or at least I don't think I am). I don't think I'm pregnant because I am on birth control and have had my period every month. The only other pain I'm having is lower back pain. Is there anything else that could cause a pain like this?

Thanks a ton!!

PS.. I've done searches elsewhere and its only come up with pregnancy cramping..