If in the colon, a person may experience abdominal pain and weight loss with no known reason. If in the rectum, a carcinoid tumor may cause blood in the stool, pain in the rectum, and/or constipation. Typically, liver enzymes in the blood will destroy hormones that can be made by GI carcinoid tumors.
Gastrointestinal stromal tumors (GISTs) start in special cells in the wall of the gastrointestinal (GI) tract, also known as the digestive tract. To understand GISTs, it helps to know something about the structure and function of the GI tract.
Anal Cancer (Rectal Cancer): Anal cancer can cause bright red bleeding and other symptoms, such as itching or a bump in the anal region that can easily be confused with a hemorrhoid. Foods and Medications. Medications or foods can also alter the color of stools.