I Have Been Having Back Problems (pain) , Just Below

I have been having back problems (pain) , just below my shoulder blades (across back), its been gradual i believe since i got knee replacement surgery (not sure that has any to do with it. Pain seems to also be getting worse as i do some usual daily activites. I have to sit down lean back on chair /couch and put my feet up that helps for a while. Will then start back up as i continue to move around. Very frustrating ! not always on going, cant seem to put "finger on" what actually,really agrivates it.