I'm taking antibiotics for my urinary tract infection and every time I take one I have back and abdominal pain. What does this mean?

Back/abdominal pain. There is no way to make a diagnosis without knowing what you are being treated for, with which antibiotic, and also necessitates more info and a physical exam. Go back and see the doctor who is treating you. Los Angeles has a plethora of outstanding infectious disease experts. If your doctor is not sure, get a consult.