What Causes Knee Pain?

A couple days ago my knee started acting up. It felt that it was going to give out at any time. I had not injured my knee before, but it's been feeling this was for 4 days so far. Should I go get this checked out? What do you think it might be? It hurts above my knee cap and I can't really walk much on it. I got a good little limp going on, but I am just curious as to what could be going on because I didn't do anything to make it feel this way.