Well the obvious question here is did you have an x-ray or MRI to determine whether it was a root left behind of a tooth. The biopsy would not remove it - it would need oral surgery for that. Cancer typically does cause pain, although there are exceptions. Infections always cause pain.

Another thing that could cause this is a crack in the root of a tooth. It will cause a fistula to form in the bottom of the root area through the gum (which starts as a pimple - when it ruptures it will drain and the pain will stop). Antibiotics will only be a stopgap measure if this is what you have The solution will require removal of the tooth by an oral surgeon.

Cold sores are yet another thing - they are a form of Herpes and can get inside the mouth as well, especially if you are under a lot of stress. You could practically get herpes just reading some of these posts - lol! Acyclovir is one possible treatment here.

Take a chill pill - I still wouldn't be looking for cancer but you are wise to rule it out early.