The dentist sewed the skin shut over the socket. Perfectly normal, it speeds the healing.
Because there was a protrusion into the sinus cavity, when the root of the tooth was removed, it irritated the sinus on my left side.
Let me pause here to give congratulations to the astute who have connected the dots already and are wincing, and to warn the squeamish that this might be a good chance to stop reading.
The irritated sinus is swelling and runny, making me feel like I have to blow my nose. When I do so, the tiny hole allows the pressure to go in behind the newly-sewn together tooth socket.
I can actually taste the pain.