Knee injury

Patient: What could possibly be causing this pain in my knee?

Symptoms: When I walk my left knee feels like the knee cap isn’t in the right position, sort of like theirs fluid under the knee cap, also when my knee isn’t straight it feels like the knee cap is grinding on top of the knee bone. Also when I stand for more than about 3 minutes my knee feels like its locking then my knee will buckle. It rarely happens in my other knee but it has before