The weight loss you specified as a prospective cause might be the reason you are experiencing knee pain on one side. On the other hand, weight changes could be a culprit. The infancy of sudden neck pain and spinal cord delineating the muscle sprain might be the cause. Following healthy weight suggestions and gentle stretching activities is important. Not doing so will put you at risk for other issues like diabetes and other ailments so I advise doing the things I suggested above first. If the pain doesn't go away, it's best to call an orthopedist to look at it.