What Causes Stiffness And Sharp Pain Under The Knee Cap?

Hello, I had a knee injury years ago and never had surgery or received medical attention. Recently, I ve been having problems if I kneel for extended periods and stand. When I walk, every now and then, it locks/gives and I get a sharp pain under my kneecap. I m considering a knee strap that wraps directly beneath my kneecap. Thoughts? Advice?