The reason for the dark red blood after stool is upper gastrointestinal bleeding, ulcerative colitis, rectal cancer, etc. 1. Some people will have upper gastrointestinal bleeding, not only will there be pain in the upper abdomen, and part of the blood will be discharged through the digestive system again. The blood stays in the digestive system for a relatively long time, so the color is mainly dark red, and some people even form tarry stools. 2. Ulcerative colitis not only causes damage to the function of the colon, and in severe cases, there will be blood leakage from the mucosa of the colon. The blood will be mixed with feces in the intestinal tract and will be excreted along with the feces. Because the blood has been partially oxidized in the intestine, the color will appear dark red. 3. Rectal cancer is a malignant tumor disease, which will lead to local tissue erosion and blood leakage in the middle and late stages. When the blood is stored in the intestine for a long time, the color of the discharged blood will be dark red.