it could just be a big poo or you have a hemorrhoid, go to your doctor..It is highly unlikely that a 16 year old would have cancer of the colon. You could have hemorrhoids, or have had bouts with constipation. Drink lots of fluids and eat lots of fiber. Constipation will cause your rectum to expand more than usual when your stool is harder and larger than normal. If this continues, please see your doctor. I seriously don't think you have anything to worry about though.