What Could Cause Swelling, Pain, And Indentation Around A Previously Operated Knee 10 Years Later?

My left knee I had surgery on 10 years ago . Now it is swelled up again and so is my leg. Whole knee is like a baseball size. I have an indentation below my knee also that keeps getting bigger . The pain is all around the knee down the front of my leg and behind my knee and goes up into my thigh on the backside . Do you know what this possibly is ?