You are probably just feeling it because the dentist pointed it out. If it were cancer, you would have sores, pain (especially when you eat citrus), swelling in your neck, along with other things. Do a little research and see that you don't have these symptoms. Chewing tobacco for so long puts you at some risk (you did quit, though, right?), so you should keep an eye on things.
I would suggest bringing it up the next time you see your doctor (you get regular physicals, right?). If you are really worried, it wouldn't hurt to go in earlier.