According to the Mayo Clinic, blood clots in the stool or fecal material are evidence of a bleed somewhere in the gastrointestinal system. Blood clots in the stool will often be accompanied by unclotted blood. If the unclotted blood is bright red, then the bleed is most likely lower in the gastrointestinal system, such as the colon or the rectum. If, on the other hand, the blood is black and resembles tar, this means that it came from further up in the system, such as the esophagus or stomach. Black, tarry blood, also known as melena, gets its color from being partially digested by bacteria in the gastrointestinal tract.
In some cases, blood clots in the stool (and the gastrointestinal bleed that causes them) can be due to certain kinds of infections, such as some strains of salmonella, and some stomach parasites. Gastrointestinal bleeds can also be caused by ulcerations, or ulcers, as they are commonly called. Stomach ulcers are actually typically caused by a bacteria called H. pylori, whereas esophageal and intestinal ulcerations are usually caused by stomach acid or other caustic substances burning the lining. Another cause of gastrointestinal bleeding is called angiodysplasia, which is a condition in which blood vessels near the surface of the intestines expand and break more easily, leading to blood seeping into the digestive system.
In many cases, gastrointestinal bleeding can be caused by conditions that are pre-existing and that will most likely not go away over time. The most serious of these is cancer, especially colon cancer. Colon cancer normally begins from small growths called polyps, which are small clusters of abnormally growing cells in the colon. Both polyps and cancerous tumors will bleed more easily because of their abnormal growth and because they disrupt the healthy tissue around them. Other causes of chronic blood clots in the stool can be grouped under the umbrella term of diverticulitis. This occurs when a portion of the intestines becomes looped around itself or blood vessels, causing constriction. Over time, this causes that portion of the intestine to lose blood flow, causing it to slowly die off and bleed.