You need to see the vet. Where do you believe it's dislocated? They are prone to luxating patella's (slipping kneecaps). But you should have it checked out to see if it's dislocated or possibly broken so that they can do whatever needs to be done for it for the future health of your pup as well as provide some relief for any pain. Ask yourself, what would you do if you were asking the question about yourself or a human family member?