frequent Urinary tract infections?

In July I got a really bad UTI, I went to the doctor and got medication but it didn't help so I continued to go in. When 3 different medications didn't work they started testing for other things. I've now gotten tested for all the STD's and they have all come up negative but I am still having severe pain. I am on a 3rd day of a medication for a UTI today and I'm incredibly discouraged because the first couple days it seemed to mellow out but now its flared up again. and I'm on medication! What can this be?? I'm worried it might be a infection in the fallopian tubes but the doc I've gone to keeps dismissing that thought and wont test me for it, but none of th UTI meds are working! I want to try and have another baby but I can't when this thing (whatever it is) keeps flaring. has anyone had anything similar...what did you do to get it fixed? I do everythign your suppossed to do, I go pee all the time, drink lots of water, pee after sex, keep it clean....what the heck?