Hi there,
My first thought is that it's the sciatic nerve being pinched or irritated in some way. Before I had my hips replaced I had pain that would start in my, um, rear end, shoot down the back of my thigh, SKIP my knee area and continue down the back of my calf. After the hip surgery the sciatic pain disappeared. My guess is that the hip pain was altering the way I walked, sat, stood, etc, which can easily lead to pain elsewhere in the body.