When it is caught early, colon cancer often does not cause symptoms. Over time, though, if a tumor is allowed to grow, it can start to cause problems within the bowel. Although there are many causes of thin stools, this symptom can be caused by a large tumor in the colon.
In an otherwise healthy person, thin stools related to colon cancer are usually caused by a narrowing within the colon, also called a partial blockage of the colon. Think of your colon as a hollow tube, like a drinking straw. If a tumor is growing inside the colon, it can start to obstruct the flow of stool through your colon, just like a pea inside a drinking straw would block or slow the flow of fluids
Left-sided colon cancers most frequently cause bowel movement-related symptoms of colon cancer, such as thin or bloody stools.
Cancers of the rectum can also cause partial and complete blockages of stool passage, as well as an uncomfortable feeling of urgency. This symptom can be caused by a growing tumor within the small rectal space, which places pressure on the nerves, making you feel as if you need to move your bowels.