What are the odds that after having a clear colonoscopy 17 months ago, a mass I have now is cancer?

High. Cancers tend to grow more quickly than non cancerous tumors. Therefore, if you had a clear colonoscopy 17 months ago, and you now have a tumor in your colon, the chance that the tumor is cancer is higher than if you have not had a clear colonoscopy 17 months ago.
Unlikely colon. From the time a polyp begins and it turns into cancer usually takes more than 10 years. A careful colonoscopy should prevent colon cancer for at least five years. Some doctors colonoscopies are not as careful. The measure that reflects the care is the adr (adenoma detection rate). If your doctor's adr is below 20 (or unavailable!) i would be concerned.