Low back pain while standing still for 30 seconds?

I had surgery for severe sciatica over a year ago. Immediately, my sciatica was gone, but also I had a larger surface area low back pain. My surgeon said essentially to suck it up because I had surgery it probably hurts still. (This was fair at the time. He did do a 2nd MRI and found nothing). I am very strong. I work out three time a week, doing yoga 3x f per week for 6months. I can do cardio for 60 minutes, I avoid spinal compression weight excercises. HOWEVER :) If I stand still for more than 20-30 seconds, I begin to freeze up. It begins to hurt, but hurts much more when I subsequently begin to move (sit, walk or whatever). It's the wierdest feeling. It's as if I am a rusty robot. The pain radiates in a large portion of my lower back. Once I am moving (10 seconds more?) it does not hurt. What is going on?