I get those right on the spot where my wisdom tooth used to be too. I don't know if they're pimples or cysts though. Mine sometimes are filled with blood. They hurt as they get bigger, and then they burst.
Are you sure its a pimple? , is it hard ? - it could be a bit of the bone in your jaw protruding through the wound , or it could even be a bit/a chip of the wisdom tooth still left in your gum.