I'm 23 and i have had the same problem since July when i did a 200km ride on a new bike that wasn't set up properly. Its a pain from the outside and left side of my right kneecap. I've only ridden a few times after that, when on SPD pedals, the pain is is really bad, but when on Speedplays it's not really an issue. The only problem is that i really enjoy CX/Gravel riding where Speedplays aren't really the ideal pedal. When walking or doing any other activity the pain is barely noticeable.
I have not been to a doctor or any physiotherapist but i guess it's some kind of inflammation. I'm going to rest for a few weeks now and then start to build up some more knee strenght over the winter. I've never been injured before and Cycling is the only activity i wholeheartly enjoy so this pain is really annoying.