What Causes Rectal Bleeding?

Question: Well, I screwed up. I was running a fever of 101 or so on Tuesday and Wednesday, with no obvious symptons, other than having to pee every half hour. The last time I had those same symptons it turned out I had a UTI. So I took a 500mg cipro on Wednesday morning and evening and then another on Thursday morning. Thursday afternoon I attended a Bikram yoga class (105 degree heated room), as is my daily norm. I could barely make it through the class and had to sit out the last 3 poses, which is highly unusual. After class I went to the bathroom, once I was able to scrape myself off the floor. I had a huge BM and then a steady flow of diarhhrea. It took me another hour to be able to get myself dressed and home. I was also extremely chilled. Once home, I had more diarhhea, and noticed that I had a considerable amount of bright red blood and tissue in my stool. This at the end when apparently there was no actual stool components left. I would say my strength, body temp and energy finally came back to normal about 3 hours after class. I am a very healthy and active (yoga 3 times a day) 61 year old female. I actually had a colonoscopy last fall, that was clear, save for 1 polyp. I feel fine otherwise - and as you might imagine, stopped the cipro. I know I should never have taken it to begin with without a new prescription - nevertheless... How concerned should I be about the rectal bleeding (I know it is never a good thing)? I am assuming it is a temporary thing from the violence of the diarhhea. Or so I hope. Please advise!