To have pain like this I would say the ergonimics (sorry spelling) needs to be adjusted either in your chair or table height. I worked in a corporate office and every six months they had a team of people come through and measure how you where sitting in your chair at work and would adjust chairs to make sure you where getting the best seat and posture possible. I would suggest doing some research online of how to have a friend or spouse observe how you are sitting when sewing to get an idea of how to change things to make it better for you.