I was prescribed Bactrim for a UTI and within one day started to ...

I was prescribed Bactrim for a UTI and within one day started to experience severe lower abdominal pain -- feeling as if my entire lower abdomen, and around to the back, was caught in a vise. Not a stabbing pain, but a pressure-pain that would not let up. The next day the pain had moved up to the mid-abdomen and had become a burning, pressure-pain that I imagine might be what an ulcer might feel like. My stomach and abdomen seem or feel swollen, too. Maalox seems to help, but only offers a little relief and doesn't last long. A dose of fish oil seemed to help a little at bedtime last night. I wonder how long this will last and what mechanism of Bactrim could possibly cause this?