diagnosis for colitis

My doc has diagnosed me with ulcerative colitis based on 2 biopsy samples taken during a colonoscopy.  However, when viewing the tissue during the colonoscopy my doc said my colitis had the "appearance" of pseudo membrane colitis.  The latter makes sense to me as I am only 17 years old and have recently taken the antibiotic Septra in fairly large doses to combat a staff infection.  Once I stopped the Septra, the colitis began.  I am currently taking flagyl, but my doc wants to start me on high does of prednisone!  What should I do?  Help!