total knee replacement

I had my right knee replaced on February 2, i am 59 yrs .. I had my total knee replacement surgery in 1995. I am now having a great deal of pain in that knee. It feels like I have a torn muscle right across the knee cap area. It hurts, burning like a torn muscle, like heck when I go UP stairs and when I rise from a sitting position. What might be causing this? please answer very urgent!!!!!!