What Could Be Causing My Lower Abdomen and Pelvic Pain Accompanied by Diarrhea, Nausea, and Loss of Appetite?

A slight burning sensation on the left side of the stomach can be caused by acid reflux, gastritis, peptic ulcers, gastrointestinal infections, indigestion, gas, or irritable bowel syndrome (Ibs). 

Many people­ get irritable bowel syndrome­, also called IBS. It can make your stomach hurt and cause bloating, loose­ stool, or hard stool. Things like stress or certain foods might worse­n it. Eating smaller meals could assist. Avoiding foods that trigger it, such as spicy ite­ms, can be beneficial too. Managing stre­ss helps a lot of people. Drinking lots of wate­r daily and keeping active may e­ase symptoms for some people.

It's possible that the wound from your radiotherapy treatment has not fully healed or that there may be other factors contributing to your symptoms. You should communicate openly with your doctor about your symptoms, concerns, and treatment history, as they will have the best understanding of your problems. 

I advise you to take your child to a doctor. Based on proper evaluation and diagnosis, the doctor will be able to determine whether the problem faced is due some underlying medical issue such as food intolerance, infection  or some other reason. Based on that, treatment may be suggested which may include certain dietary and lifestyle changes, medications etc.