Runner's knee is a very common injury that most runner's will experience once throughout their running career. Pain behind the kneecap, popping or grinding sensation in the knee, experiencing pain while running, bending the knee or walking may all be symptoms. If these appear to be any of your symptoms, you should definitely get to a doctor to get it checked out. Resting the knee, compressing it, icing it and elevating the knee are all ways for you to treat it yourself.