Weak thigh muscles, a traumatic injury etc can be the cause of your patella disorder. If your knees experience swelling and severe pain a complete dislocation of patella can be the reason. As you doubt weakness of vmo muscle can also be the reason for your knee pain. Care should be taken since vastis medialis weakening can cause more trouble including Pattellofemoral pain.My mom suffered from similar condition of pattella dislocation and since it wasnt correctable she went through a surgery for that. She is continuing with her massage therapy sessions even after a year of her surgery .