carterlor is right. Endometriosis is a good candidate for your pain and since it doesn't show up on tests it's very easy for doctors to overlook it. Once you've exhausted all testing, a diagnostic laparoscopy would be the last thing to see what's going on in there.
I have had since I was 16yrs,I am 35yrs now-& I have had a history of endometriosis!It is VERY painful & has to do w/ period's,i believe this may be your problem. The ONLY!! way they can "see" it is by labrascopic surgery-bring this up to your Dr. but alot of time's they think it's all in your head!!please check w/ Dr. about this,it's overlooked alot of time's!!good luck!!