Persistent Burning And Increased Urine Frequency Post UTI Treatment, No Relief With Doxycycline. Any Insights?

Hi, I am a 23 year old male. one week ago I had a UTI . white blood cells in urine. Doctor assumed sti or uti and prescribed zithromax . One dose of 2 pills taken at once. It rocked my gut and gave me diarrhea . I do suffer from mild IBS as well. The next day, after z pak, the pressure and urgency increased. Burning sensation in urethra got a little better. 4 days later went back to my student health clinic, with persistant symptoms, urinating a few times an hour. Another urine test showed no white blood cells. He prescribed me doxycycline 100mg for 7 days, suggesting I wait 2 days to rest my intestines. The doxyclycline hasn t relieved the frequency, and mild burning. I have lower abdomanal pain and I thought it was my bladder but I am wondering if it is my intestines. Can antibiotics cause gastritis? Could this be the reason for the uti symptoms without infection? When I sit on toilet I do seem to relax and get some relief from the sensation. Even though it has only been a week, it feels like eternity and the symptoms are starting to take a mental toll. Any help or insight will be greatly appreciated. Thank You, Ben.