I don't remember my abdomen getting hot. Even though I don't have much blood and mucus, there's NO doubt that it's not IBS. My C-scope confirmed that...it would have been "unremarkable" if I had IBS and not UC. Instead, my entire colon was inflammed, but I only had very, very small ulcers. Maybe that's why I didn't have much blood and mucus. Or maybe I did have mucus and my diarrhea was so liquidy it was really hard to see. At any rate, I also feel that I don't have IBS because food doesn't seem to make any difference, and because I don't seem to go back and forth between good and bad days. Another reason I know I have UC is because I responded to the mesalamine drugs and to prednisone. Neither would have helped IBS.
I also don't feel that the amount of blood equates to how bad the flare is. At my worst, I had 20 or so extremely urgent BMs/day, and was 25 lbs below my pre pregnancy weight (and I'd gained about 25 lbs during pregnancy) by four months after I had my baby.