What Causes Stiffness And Swelling In The Knee After A Replacement Surgery?

I had ACL replacement surgery almost two years ago and did all of the physical therapy that my surgeon recommended. However about a year ago I started having trouble straightening my knee out all of the way and now it is even worse. I have severe pain when I am standing and working on my feet all day. My knee swells at night and in the morning I have a lot of stiffness. What could be wrong?