There are a multitude of reasons for back pain when it comes to Steadicam. Vest set up, balance of the rig, adjusting your socket block screws properly, where your vest sits on your body, how you stand and move. What kind of vest do you have?
Also, do you have an exercise program to strengthen your back and legs? Stretching, yoga, physical therapy and massage are all things that can alleviate back pain. I switched to the Exovest and have almost zero back fatigue. Feel free to PM me with any more questions.