Having rectangle stool. had clean colonoscopy 20 months ago with good prep. what are the chances colon cancer would be causing this?

Low Chance ; BUT: At 40, a colonoscopy which shows no cancer less than 2 years ago is reassuring. Regardless, ASK your doctors advice about these new symptoms. Anorectal diseases such as hemorrhoids and fissures can cause changes to bowel caliber, but a thorough evaluation of the area needs to be done. While colon cancers are uncommon to grow in such a short span, it is not impossible. In short, see your doctor!!