I would seriously consider buying some alcohol and swabbing before a shot. Not cleaning the injection site before and after a pin is just lazy and bad practice.
So are you saying that every doctors office, hospital, 1st world med school is wrong by using alcohol pads/alcohol swabs before injections?
Sound like something to do with the actual pinning. Maybe hit a nerve, vein, scare tissue, who knows. I'd use something to clean your skin before injection. That may have something to do with the pip. When something like that happens, it always makes me nervous when I pin again. All the above have happened to me at some point but it's just part of the game. If it was the gear it woundn't go away that quickly. Let us know how the next pin goes.