Update: I'm feeling a lot better. Still very stiff, and a little bit of knee pain (not limping, though) and I'm down a few advils a day. I even went on the exercise bike for a few minutes (not exercise so much as a test, with the pedal resistance dialed waaay down).

Thanks to everyone for the advice (including the BFer who PMed me about this).
I'm still gonna try to go see a doctor about this, though, to check when I can get on with normal cycling.