Polyps, non-cancerous colon masses, as well as irritable bowel syndrome can cause a change in stool shape. Without a family history, cancer would be less likely in someone who is 39 years old.
The colonoscopy is the best test to rule this out, and I would certainly encourage that you follow-through with this test. Other than cancer, this test can evaluate for polyps, diverticulosis as well as inflammatory bowel disease.
If every test is negative, irritable bowel syndrome can considered and treatment (i.e. antispasmodic agents) optimzed.
Followup with your personal physician is essential.
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, M.D.
Medical Weblog:
kevinmd_b