Maybe this is a hyperextension? Or an ITB issue? Or something completely random?
The knee hurts when I poke it from the outside sort of in the space where the joint would be. It hurts to walk normally when I pick up my foot (i.e., bending the knee), and it hurts to go up and down stairs.
Judging by how quickly this seemed to resolve, I'm thinking this was a hyperextension, and rest and treating it nicely for a week or so seemed to have been key!