I had right knee pain on my first with the LHT. SPD pedals. I just pointed my saddle 2 degrees to the right and it cleared right up. On a longer tour the pain returned. I noticed that with how my foot liked to fit on the pedal, the spd cleat was twisted in the pedal. I adjusted the cleat and the pain went away! Perhaps your knee is used to being held in place by a clipless pedal and now, since you're on platform pedals, it is pained from the different motion.