Thanks for the advice everyone - I will follow up with my fitter. I've also noted knee pain on my BMX pedals occasionally - probably when my legs/feet are in the wrong position. Interestingly enough, it is the OTHER knee that seems to be giving me problems recently, but it passes when I change position - and once I get warmed up that is the end of it.

I think part of it is "spring knee". Kind of like "spring butt".