It could be arthritis, or some other joint-related syndrome. Do you live in the cold, damp wintery north? The dampness doesn't help with joint problems.

The other thing too is that you might be putting pressure on one particular knee because of your gait. I have PD mainly on my right side and tend to put my pressure on my left leg. As a result, my left knee and foot tend to bother me more than the right side even though I had extensive surgery on the right foot due to a clubbed-foot correction.

Hope this is something that helps for food for thought. If you're in a lot of pain, see if you can contact an orthopaedic specialist about it.

John