swollen gum

Hi all,

I have a hard swelling around the root area of my UL7 molar. It feels like it's about 1cm (guess) in size. About 10 years ago I had root canal treatment on this tooth, and had a crown placed over it. I doesn't really hurt. I went to the dentist on Friday for a check up and x-rays. The thing that I am worried about is that after the check up and x-rays the dentist didn't say anything about the swelling. Is there a chance that he may have missed it or something? What could it be and the possible cause?

Really would appreciate a response!

Thank you.