I didn't go to medical school, and I stayed at a KOA last night, but if I had to guess, I'd say you are riding somewhat rigidly in one position, and the combination of the new effort and some femoral nerve pressure are causing your symptoms. The fact that it's both knees and it clears up in seconds leads me to think it's not serious, but I had a rustic site with no electricity far from the head so check with your docs if it persists...