Three Episodes Of A Lot Of Blood While Having A

Three episodes of a lot of blood while having a BM. First one occurred about three months ago and only happened twice. About a month ago - lot of blood with each BM for a week. Colonoscopy was negative. Been experiencing lightheadedness for the last two weeks and now again today - two BM's with blood. No leakage at other times. Not constipated - usually soft stools.