What Causes Blood In Stools After Treatment For Rash On Body?

I am a healthy 22 year old female and last week I broke out in a rash and/or hives everywhere. I went to the doctor and he said it was either an allergic reaction to something or a rare virus... but he didn't say which one. The rash/hives were in clusters, very red, and raised up. It started on my hands and by the end of the day it was all over my arms, legs, stomach, neck, back, and chest. I tried hydrocortisone cream, oatmeal baths, and cold showers but it didnt help. The doctor also prescribed Pretisone, a steroid, but it didnt help. The only thing that helped was taking a lot of Benedryl. The rash/hives are gone now, but I recently had a bowel movement and had a significant amount of bright red blood in my stool. Could something seriously be wrong other than an allergic reaction to something?? Thanks!