At this time, I would proceed with the colonoscopy. This is the most comprehensive test to evaluate for colon cancer, and can also look for colitis and other forms of inflammatory bowel disease.
Other possibilities include malabsorption, Celiac disease, and lactose intolerance. Tests to evaluate for these conditions include sending the stool off for analysis, obtaining antibody tests to exclude Celiac, and going through breath tests to look for lactose intolerance.
These options can be discussed with your personal physician.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin Pho, M.D.
KevinMD.com
Twitter.com/kevinmd