A lot of bleeding is not common after a colonoscopy, according to the American Cancer Society. A large amount of blood is potentially dangerous, and continued bleeding or blood clots should be reported to the doctor, Mayo Clinic advises.
Other warning signs that require medical attention include serious abdominal pain, nausea and vomiting, the American Cancer Society explains. These signs sometimes indicate that the colon or rectum was punctured during the procedure, which puts the patient at risk of a serious infection. It is normal for a small amount of blood to be in the first stool passed after the colonoscopy. Other typical but harmless effects of the exam are bloating, gas pains and cramps.