Hi woody,
It seems you are suffering from patellofemoral pain. The cause of kneecap pain might be injury or overuse which may have caused weakening of the cartilage under the kneecap. To prevent future instances of knee pain, you must take care of your knees and resort to low-impact exercises. As a physiotherapist working with the top orthopedic group long island I would recommend you to immediately consult a doctor before the pain gets critical and unbearable. You might be in need of a specialized workout program to treat the damaged knee.