perose, I think you should call NHS direct right away! Kazza is right! I am going in to have both knees replaced soon and have been told that infection is the biggest danger in joint replacement. You may well have an infection deep in the tissue around the joint that is causing the tenderness. This isn't always visible from the outside. Please do not wait till it gets worse, but see your doc asap. Call NHS direct today though and get advice. Hope you feel better very soon.