I'm not saying this is what's happening with you, but I had similar symptoms in August and think the culprit was Rowasa. I was trying Rowasa one night, generic the next and nothing the third. I still had bleeding on some days.
Fortunately I keep a journal and I was able to see that the bleeding came the days after I used the Rowasa. I quit using it and haven't seen any blood it weeks.
We're all different, so it's really a good idea to write everything down and be your own detective.
Potato... Female, 58, dx 11/07 mild left sided ulcerative colitis (50 cm)
asacol 3 3x a day, cortenema (generic) every other night
culturelle, benefiber wafers metamucil wafers, activia yogurt, spinach & sunflower diet, primadophilus reuteri
children's vitamin, Vitamin E 400 mg, omeprazole 2xday
oxazepam when needed, lomotil or ImodiumAD when needed
Yoga, meditation, The Work by Byron Katie, exercise at least 5 days a week