I rode my first few thousand miles without knee issues either. As I got stronger and started putting more power to the pedals, climbing more, putting in more miles I began to have knee problems where none existed before. Everyone's feet, ankles, knees, and hips are connected differently and most in a less than a perfect manner. Getting the load balanced across your knees both anterior/posterior and lateral/medial is key to making the joints last as long as possible. I'd highly recommend seeing a good fitter as you are already experiencing pain. I was surprised at how dissimilar my leg lengths were and the amount of varus tilt in my feet under load that was discovered during my assessment. My results have been good so far and honestly I would have had to quit if not for getting my knee load and tracking "fixed"