how to clean out my colon?

I went to the hospital for horrible stomach pains in January. They sent me home with something minor. I went back in July for worse stomach pains (just above my belly button) and they said I had 4 feet of stool in my large intestine. I still am constipated and it’s now almost September. I think I may have a tumor or colon cancer. I want to clean out my colon and get rid of all the poop to see If my stomach still hurts and get another xray. What’s the best way to do this?