My knee quite often does that, and it started as suddenly as you describe. When I went to the doctor, she said that yoga might help. But now I've raised my bicycle saddle to the correct height it seems to have stopped happening, so I'm hoping it was that.

I think an excess of religion and thespianism might have been contributory factors - I never got it before I went anglo-catholic and consequently had to kneel and genuflect a lot, and it began just after Titus Andronicus in which my character fell to her knees an astonishingly large number of times.

I'm not, of course, suggesting that cycling, religion or acting caused your problem (it would be very strange if they did), but you should perhaps be looking for knee-strainable things you do all the time as well as things you've just done once, recently.