Colon cancer may cause changes in stool, such as blood, narrow “pencil” stools, rectal bleeding, diarrhea, constipation, and abdominal pain. In advanced stages, symptoms can worsen, including dark, sticky stool (melena), nausea, weight loss, and bowel obstruction.Other signs include feeling unable to empty, your bowels (tenesmus), alternating diarrhea and constipation, and fatigue due to anemia. While these symptoms can also result from other conditions, they should always be evaluated by a doctor. Early detection is crucial, as colon cancer is highly treatable in its early stages. If you notice unusual or persistent changes in your bowel habits, seek medical advice promptly.