What Is The Cause And Treatment For UTI?

I have had 2 sets of antibiotics for a urine infection. I have had all the symptoms very severe but urine test results show no bacteria.
Still have symptoms my doctor don't know why ....he's put me on 3 months of antibiotics! Yet 2 weeks of them hadn't worked yet.
I've had this for over a year since I had surgery .....13 cases of apparent water infections ....I'm in agony and my bladder hurts all the time, is there another reason as it can't be water / kidney infections with no bugs in urine and 2 weeks of antibiotics not worked ...