I've had a recurring UTI, and have been on two different types of antibiotics 3 times. what's going on?

at first, my GP thought that my urine was just concentrated causing the burning pain since a quick office test came up negative for a UTI. after a few days the docs office called back saying i came up positive for one after more tests. the first meds (cefprozil) helped for a week or so but the pain came back. after seeing the doctor again, he prescribed the same meds for another 5 days. after it not helping AGAIN, i went to a urologist who put me on macrobid. that finally helped, but now a few weeks later i'm feeling lower back pain on my left side and frequent/painful urination. i've been drinking a lot of water and taking cranberry supplements as well as acidophilus, which has helped clear the cloudy urine. i'm really tired of having this happen all the time, especially since i'm aware of the fact that i'm susceptible to UTIs and drink plenty of water and take proper vitamins. am i on the right track to getting better, or is this another UTI? could it be something more serious?