Hi, I used to experience this, too. It got so bad, at one point, that I had to stop knitting altogether. Cause was the 'throwing' of the yarn repetitively in my right hand. Taking breaks and stretching helped a bit, but not for long.
I had the good fortune of having someone show me 'continental' style knitting where the yarn is held in the left hand and hands basically hold almost stationary, with no throwing of the yarn around the needle. Like magic, all my shoulder and back pains went away. You might try it and see if it helps you. One cool benefit of learning this method once you already know american style is that you can easily switch hands, whenever needed, and two-color knitting is a breeze... just knit one color using american, the other color using continental.