I agree with kcdc, you need to see an ENT for a look. The dentist is definitely wrong in one area... cold sores, (herpes simplex) do not last for long periods. They resolve, on their own in 10 to 14 days. So I wouldn't trust this persons opinion if he made this comment. There are plenty of benign things that occur orally and may look like what you describe, but please err on the side of caution and have it looked at by someone in the know. On the positive side of your comments, the fact that it comes and goes is not like oral cancers which do not behave this way.

Brian, stage 4 oral cancer survivor. OCF Founder and Director. The first responsibility of a leader is to define reality. The last is to say thank you. In between, the leader is a servant.