If you're knitting somewhere where you don't have good back support, then that could be causing problems, too.
Do you have a recliner or a couch you can relax and knit on. I almost always knit in my recliner. Of course, that doesn't automatically mean I'm relaxed (I still have to tell my shoulders, "DOWN!"), but I'm much more relaxed there than, say, sitting on my bed with no (or poor) back support.
Hope something will work for you!