I had a root canal done on a back molar 4 years ago. It's always been a little sensitive and not "dead" feeling like another I have. Also, when the root canal was performed one of the files the dentist used broke off in my tooth during the extraction of the nerve. Now my tooth is EXTRA sensitive and has a swollen area on the gums/bone under this tooth. What can this be and do I need to see a dentist immediately? If so, I obviously won't be going to the same guy again!!

Definitely get to a dentist. It sounds like the tooth is infected again. Try to see a root canal specialist if possible.