So I'm walking today at work on a normal flat-surface floor at a slow-ish pace, didn't take a mis-step - nothing at all abnormal. Got a sudden sharp pain in the distal-medial patella. That was at 9 this morning, it's 4 now and it is ok as long as I don't move my leg

. It's gotten more sore every time I walk, and now I'm limping pretty significantly on it. It's not discolored / bruised, I can't decide if it's swollen or not (so not badly if it is), and it's not tender to palpate.
I've iced it, but... HUH!?!?
How does one hurt their knee just taking a completely normal step that is exactly like the ones before and (theoretically) after?
More importantly - any thoughts on what I did? I didn't feel a snap or crack or anything - just sharp-ish pain that I thought would walk out in a step or two - but no.