I work with special needs kids. Amongst them are a couple of teenagers who are in a wheelchair. The lack of movement gives them all sorts of aches, pains, and discomfort. I always make sure that they have to move (within their limitations) about every 20 minutes, and if that is not possible, then a gentle massage of the muscles that have tightened often helps. Physio-therapists will tell you that the best position for your body when you are sitting down is a straight back, a 90 degree angle between your back and your upper legs, another 90 degree angle where your knees bend, and your feet should be flat on the floor. I bet a lot of us are not sitting in that position when using the sewing machine. Check it out. You may have to adjust the height of your table, and/or the height of your chair. IKEA sells tables that are height adjustable and not too expensive (if you haven't got a husband whose diy skills are magic), and most office chairs are height-adjustable. But even with that sorted out, you need to take regular breaks. I'm my own worst enemy and can get very engrossed in doing something. I sometimes use an kitchen timer and set it for 20 minutes at a time. When it bings I get a drinks, empty the rubbish bin, tidy up some fqs, get my bag ready for school, etc. Works for me.
And go easy on the pain killers. They only mask the problems, not solve them.
Lorchen