HI there and Welcome to Ulcerative Colitis,
Im not going to pretend to be an expert as I am just helping out but I have done a little research for you. (Anyone, please feel free to correct me if I am wrong here!)
5-ASA = Mesalazine = Anti-Inflammatory Drug. I dont know what you call it in the US as you use trade names and I am from the UK where we use generic names (Possibly Asacol or Ipocol) Are you allergic to that? Have you been using it and the doctor has taken you off? When you doctor said about something irritating your colon, my best guess is that it was either the result of the allergic reaction or a food type you are eating that your bowel does not like (Foods vary from person to person). Prednisone is a steroid used in the short term for UC but there are alternatives. Im not sure what they are (hopefully someone will rescue me here!) but I know they exist.
If you have any other questions, please ask. There is no such thing as a stupid question if you dont know the answer.