5 months after rectal cancer surgery, always have diarrhea during radiotherapy, the reason can be radiotherapy side effect, gastroenteritis and other factors. Rectal cancer is a common malignant tumor of intestinal tract, which is mostly caused by bad dietary habits, heredity, gene mutation and other factors. One of the main therapies after diagnosis is radiotherapy, which is mostly used as postoperative adjuvant therapy. Common side effects during radiotherapy include gastrointestinal reactions, which can be manifested as diarrhea, abdominal pain, and secretion of more mucus. If the radiotherapy site is low and close to the anus, anal swelling and discomfort may also occur. In addition, when there is a bacterial infection, eating unclean food, etc. can lead to gastroenteritis, and then diarrhea performance. Patients should consult a doctor in time to clarify the cause of the disease.