I have had the same problem for the last 5 years. I'm 24 years old and have taken so many antibiotics long term that my body has become resistant to all of them and they don't work anymore. I just recently got married and this problem I have had makes it a little difficult to want to have sex with my spouse who is bothered by it.
I've had all of the horrible tests that you guys have had and my urologist told me that it could be that my bladder is rejecting my urine so he put me on vistaril (an antihistamine) and amitryptilene (an old school anti depressant but actually helps with pain more). These are very cheap meds and they have worked for quite a while but now, they are not currently working anymore.
It seems that I have to down bottles and bottles of water just to make the pain go away.
I love when I go to the doctor and they say, "do you take cranberry pills?, to you urinate after you have intercourse?" Generally my answer is something along the lines of "Cranberry pills and sex stopped helping years ago for me. years ago