Have had blood in stool off and on for almost a year.just had colon checked,he found nothing. what are the possibilities?

Blood in Stool: i assume you had a colonoscopy, which was negative. If it was a complete examination, you are unlikely to have cancer, but conditions such as diverticulosis, angiodysplasia, or even a collapsed hemorrhoid, can be hard to detect. If it was bright red blood, it is likely colonic in origin. Black stools usually indicate a small intestinal or stomach origin. If you are still symptomatic, follow up.

Local bleeding: Most common is a mechanical problem, hard stool / hemorrhoid / small fissure. Trust a colonoscopy was also performed and this will rule out amebiasis, ulcerative colitis, Crohn's, a nasty precancerous polyp, shigella, ischemic colitis (something the last one may be missed). Best wishes.

Probably not Ca: The presence of blood is only of importance when scoping reveals an associated cancer. If non is found, very rare for a small hard to detect tumor to produce blood in stool unless colonoscopy incomplete. Other sources for intermittent bleeding are internal hemorrhoids, fissue, AV malformation which can be small and bleeding intermittently. If it progresses in frequency and amt. then rescope.