No bm's for days;when bm comes, stool is hard at first, turns ribbon-like, then very soft, and then bright red blood. many trips to toilet. colonoscopy showed only benign polyp. what could cause this?

Constipation: What you may have is hard stools causing bleeding from internal hemorrhoids. You need to talk to your gastroenterologist and review your diet, eat more fiber and take stool softeners. At your age bowel habits change so do you.

Rectal fissure: These symptoms are those of an anal or rectal fissure. It might not have been visible on the colonoscopy if it was not bleeding at the time. Have your GI doctor check you with an anoscope. If s/he refuses, feel free to seek a second opinion.