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

More likely not. Generally, one would not expect to find a mass in the colon so quiclyy after a colonoscopy. If this was an accurate exam, it would be unlikely. It is thought a polyp can become cancerous over a 10 year span.
Probably not, but. The sensitivity of colonoscopy, on a national level, is estimated to only be, at best 90% (low 90's if you really want to argue the numbers). Some argue that it is as low as 80%. What does this mean? If there is a significant mass in 100 patients, 10 will be missed. And not because of foley, it is an imperfect test, yet the best we have. Speak with your doctor. A mass, anywhere, equals see doc.