Don't know if you are still checking in on this thread...Have you had any luck with finding an answer?
I have recently developed the same problem, with a deep, aching pain in my uterus...My gyn thinks it is Pelvic Congestion Syndrome, which is basically varicose veins in the pelvis.
There are varied opinions on treatment, because often the "cure" is worse than the original problem. I am taking Advil beforehand, and this helps.
Good luck!