I am 72 years old and was diagnosed with Parkinson s Disease 4 years ago. A year prior to that diagnosis I had surgery to remove a large bone spur on the back of my right heel. Because the repair involved the Achilles tendon, I had to be non-weight bearing on my right leg for 8 weeks post op. After the recovery I started experiencing a sudden blinding pain In the back of my right knee when I tried to kneel. It happens rarely but when it does I am unable to move for a few moments because of the pain. Any ideas about what could be wrong?