Pelvic pain on right side with fluid

I have had a pain that started at the top of my right hip and settles deep in the pelvic area. It is different than a sciatica pain, I've had that for years. This pain starts on the level of a bad menstrual cramp, only not in the uterus, and over a few weeks becomes so intense that I can hardly stand up or step on my right leg. Then when I think I should go to the hospital I feel a soft pop and the pain lessens.

I have gone to my Gynecologist and she did an internal ultrasound, which revealed a large amount of fluid in the pelvic area on the right side. She did not think it was a ruptured ovarian cyst, as I am postmenopausal and if it were a cyst it would have to be the size of a football to produce this amount of fluid. She than sent me to a Gastroendocronologist. I had a CT scan - it looked normal - a colonoscopy, it was normal.

My muscles and tissue in this ares is not tender. Putting pressure on the area doesn't bring on the pain. The pain resides deep in the pelvic area next to the hip bone, almost in the same place as the large colon.

It has been 6 months since my first bout with this pain and now it is starting all over again. I don't know what kind of doctor to see or what to ask. I can't sleep or exercise because of the pain, which can make a person crabby and fat. =0(

Could this be arthritis or bursitis of some kind?