I don’t understand what a colonoscopy would do to help, but an endoscopy, which examines the stomach and upper gi tract might reveal the problem. During this procedure they will take some samples to be sent to the lab. This could possibly reveal an ulcer, or any number of stomach issues. I think this would be a good place to start. The endoscopy is painless other than a slight gagging feeling. I have constant, bad stomach pain as a result of my stomach working too slowly. This causes irritation of the stomach lining. I was prescribed domperidone and I have to take it every day. It has solved my stomach pain.