What Causes Recurring Pain In The Knee?

My knee has been bothering me for a while now and recently i was running and felt a sharp pain I limped around for a while and then felt an air bubble this morning that could mover around the side of my knee. It has been sore but at times I can walk around with no pain especially when I wake up in the morning, however suddenly I take a bad step and the pain is back, what could be causing this?