In all likelyhood it is nothing serious. Two things both of which are pretty obvious, firstly you should stop smoking no matter how many you smoke and secondly you should see an oral surgeon to get a definitive diagnosis. If it is serious which I doubt very much it should be treated asap. From your description it is not the way cancer presents itself--that being said get it looked at.
firstly, most likely it is not cancer but rather a "mucocele", which is basically a saliva filled bubble. they are common. If you get another one, go to your dentist and have him or her look at it if not for anything else than to reassure you. secondly, contemplate how scared you are about cancer, and think about this--- smoking kills people. it gives people lung cancer. when you are given a diagnosis of lung cancer, you're most often dead within a few years, if not months. You don't get better from it. quit