If I had a normal colonoscopy 17 months ago but now have a mass, what are the chances that the mass is cancerous?

It really depends. If one does have a history of prior cancer, or inflammatory bowel disease or familial adenomatous polyposis or a strong family history of cancer - the mass has a higher chance of being cancerous. Always assume any new mass is cancerous and seek medical attention promptly.
It really depends. If you are a normal healthy person with no significant medical history and a completely normal colonoscopy in the last two years it is rare to develop a mass that is cancerous. That being said, colonoscopy is only 92% sensitive - that means it can miss a cancer which can then grow. It is not a perfect test. Bottom line - regardless of the previous colonoscopy - always assume a mass is cancerous.