Is it harmful to have an anal bowel movement when wearing a Colostomy due to cancer? Should I be concerned or ask my doctor about this?

If the colostomy is new then it is common to have a small bowel movement from feces that had remained in the bowel below where the surgery took place for up to a month or so after the colostomy. This will stop as the remaining bowel empties completely. If this is not your situation then give your oncologist a call. Yes. I would ask the doctor. While it is probably normal to expel something from the colostomy surgery, my understanding is that it would not be possible to have a "true" bowel movement once a colostomy has been put into place. You should call the doctor to make sure everything is fine. You have nothing to lose if you find out it's normal. The rectum that is left down there still will produce mucus and you can continue to pass this especially if your resection was higher in the colon.