Dry sockets - really painful and unreasonably so. Usually lower teeth and usually molars. I'm afraid some people are prone to them - sorry! Avoid. Seems to be due to the blood clot in the socket being lost for some reason and the bare bone exposed. Treatment is simple though: wash out the socket and pack it with a witches' brew of some sort, which often works at once. Antibiotics are supposed to be inneffective but on the other hand they seem to work.