Have you been tested for Chrons disease/IBS? Celiac? Gluten allergy?
Have them measure your fat soluble vitamin levels, provide a stool sample and have them do the full work up including vit levels, pancreatic enzymes etc.
what about an endoscopy- have you had one of those in addition to the sig and the colonoscopy? They can also often obtain a tissue sample/ biopsy at this time to test for allergic reactions/ tissue damage etc.
Its less likely to be colon cancer than other abdominal issues. Good luck! If you don't get an answer, keep trying, keep asking questions, find another doctor and bring him all your reports until you get help- be your own best advocate!