Firstly please try to write in normal sentences. Also please give a bit more detail. No one can answer much to 'I fell over, I have a pain in the back of my knee. What's wrong'? It could be anything. I suppose the good news is that your x-ray shows all is still in the right place. I would consider this a knee injury that is independent of your knee replacement and not worry about it to much in view of that replacement. Then if the physio isn't having any success perhaps they could tell you, and you us, what they feel might be going on?