There are many reasons for late-stage colon cancer patients to suddenly bleed a lot in stool, the more common reasons are bleeding caused by rupture of blood vessels at the lesion of colon cancer, bleeding accompanied by anus-related diseases, bleeding caused by inflammation of intestinal tube after chemotherapy. 1. Bleeding from colon cancer foci: when dry stool passes through the foci, the mucous membrane of the foci will be damaged and lead to bleeding; when the cancer foci invade the blood vessels in the mucous membrane of the intestinal tube, it can lead to rupture of the blood vessels and cause bleeding; and when the foci grows too fast, it can also lead to necrotic bleeding. 2. Diseases related to anus: for example, hemorrhoids, anal fissure, etc. If late-stage colon cancer patients suffer from constipation or diarrhea, it may lead to aggravation of hemorrhoids and anal fissure, and then bleeding in stool. 3. Bowel inflammation after chemotherapy: patients with advanced colon cancer need to undergo chemotherapy, which has more side effects, weakening the resistance of intestinal tract, thus easily leading to intestinal infection. The mucous membrane of intestines will show pathological changes such as congestion and edema, which in turn can cause bleeding. There may be other reasons for sudden heavy bleeding in stool in advanced stage colon cancer, so it is recommended that patients should seek medical treatment in time and complete all auxiliary examinations in order to clarify the condition, and then professional doctors will give corresponding treatments according to the condition.