You're not the first one to experience this. I have a friend (early 70s) who had her knees replaced. Over a year later, she is still experiencing pain. Like you, the docs aren't sure why. My buddy who had knees and a hip replaced, said the pain from the hip replacement was the worst of the two and lasted quite a long period of time. Possibly six monthe or more. It finally abated and he's pain-free now. How long has it been since your knee replacement. Perhaps you'll just have to play the waiting game.