knees

[SadieKate]

There are so many reasons that a tome could be written. Bike fit, cleat position, q-factor, chronic muscular-skeletal issues, genetics, skeletal structure, pushing too big a gear, etc. Nobody here can give you an answer, only a bazillion guesses.

I'd read Andy Pruitt's and Arnie Baker's books, get multiple someone's to check your bike fit and talk to an orthopod. You may be stuck with it (I am with chronic chondromalacia) but you can't hope to fix the problem if you don't know what it is. And no one here can diagnose over the net.