One of the most common signs of colon cancer is a change in bowel habits. This ranges from constipation or diarrhea to changes in the size or shape of bowel movements. A change in stool color, particularly black or tarry stools, can indicate bleeding from a tumor that lies deep in the colon. Other symptoms can be harder to pinpoint, such as abdominal pain and unintentional weight loss. Finally, some tumors bleed a small amount over a long period of time, resulting in anemia (low red blood cell count) that is picked up on blood work.