Mystery Ankle Pain

What the heck?!

I'm sitting at my desk this morning, feeling pretty good, and I go to get up and there is this pain in my ankle, quite severe, running along the back of my ankle when I try to bend it up. That was three hours ago and it still hurts to bend that way. I have no idea how this happened or how long it will last, but its threatening my Jiu Jitsu class tonight, and any other physical activity for the rest of the week.

I'd be less pissed off about it if I knew what happened.