I had a full l (left) knee replacement May of 2012 & just had a check up about a month ago, with no issues on any xrays. Now I have a large, hard lump in the back of my knee, at the right lower bottom part of the knee. My hamstring has been bothering me ( just thought it was the weather/ Osteoarthritis)today. What could it be ? It can't be a bakers cyst, don't have any fluid left in the knee, it's fake, any ideas?