Blood in the stool after 15 days of colonoscopy is usually considered to be related to colonoscopy damage to the mucosa, colonic ulcer, proctitis, hemorrhoids and other factors. 1. Colonoscopy damage mucosa: intestinal mucosa is usually very fragile, colonoscopy will often damage the mucosa, if you do not pay attention to the diet after colonoscopy, resulting in mucosal stimulation of blood in the stool again. 2. Colonic ulcer: when there is an ulcerated surface in the colon, the ulcerated surface ruptures and bleeds, and then there will be blood in the stool. 3. Proctitis: the rectum is the closest to the anus, if there is inflammation in the rectum, when stimulated, the inflammation area will bleed, resulting in blood in the stool. 4. hemorrhoids: when the stool is too dry or defecation is too hard, resulting in hemorrhoidal rupture of the nucleus of the hemorrhoids caused by the symptoms of blood in the stool. There may be other pathological factors that lead to blood in the stool 15 days after the colonoscopy, so it is recommended to go to the hospital as soon as possible and give appropriate treatment measures under the guidance of the doctor.