Belly pain after radiotherapy may be caused by intestinal spasm pain, swelling pain, gastrointestinal adverse reaction, constipation and so on caused by radiotherapy, and it may also be caused by pelvic infection and so on. 1. Intestinal spasm pain: if the radiotherapy site is connected with the intestine or mesenteric tumor, there will be intestinal spasm pain after radiotherapy, which is manifested as belly pain after radiotherapy. 2. Swelling pain: radiotherapy for tumors in the abdominal cavity will cause tissue edema, resulting in local swelling and pain, and the phenomenon of belly pain after radiotherapy will also occur. 3. Adverse gastrointestinal reactions: some side effects will occur after radiotherapy, including abdominal pain, diarrhea and other adverse gastrointestinal reactions, which will also be manifested as bellyache after radiotherapy. 4. Constipation and abdominal pain: during radiotherapy, patients will experience symptoms of loss of appetite, water intake will be reduced, coupled with physical weakness, resulting in insufficient activity, constipation may occur, resulting in abdominal pain, which is also known as belly pain. 5. Pelvic infection: If cancer patients have the symptoms of pelvic infection, the symptoms will be aggravated in radiotherapy, and they will have bellyache after radiotherapy. Belly pain after radiotherapy is generally considered to be caused by a variety of reasons, it is recommended that patients go to the hospital as soon as possible to review, cooperate with the doctor for examination, to clarify the causes and take corresponding treatment measures, do not take medication on their own.