Pain in knee cap when going up and downstairs and sometimes at rest years after meniscus surgery? What could it be?

Knee cap issues. The symptoms you describe are indicative of pain originating in the cartilage on the underside of the knee cap (patella). This condition is called chondromalacia patella. Treatment involves ice packs and nsaia for the pain. Hamstring stretching and quad balancing exercises help correct the problem. Cartilage heals slowly though. Weight loss can help tremendously.