In article ink.net,
"Emily" wrote:

It might be ITB (ileotibial band) syndrome, which I also got during a
century in May - also to the outside and below the left knee. I saw a
sports medicine doc, who diagnosed it, and a PT, who gave me exercises to
stretch it. Turns out my ITB was much tighter on my left side than my
right. It was also recommended that I lower my saddle a bit, which I have.

The pain went away gradually and was completely gone about 4 weeks
post-century. I eased back on my riding and rode easier gears until then.
It's fine now, but I am keeping up with my stretching just in case.

A web search should point you to plenty of info - just look for "ITB
Syndrome". It's common among runners, but cyclists get it too.

Good luck!
Emily

Yes, that turns out it was ITB. I saw my doctor right after that and got
some exercises and stretches to do. The good thing was I didn't have to
stay off the bike, instead just stick to the flats (hard to do entirely in
this city) and low gears. I also thought it was "runners knee" but yep,
cyclists get it though not as often. I will make it a point to do those
stretches every time I ride, no matter what.