Hi Jazzzz8,
Although I don't play Sax, I do have some fundamentals to mention on pains as such.
1. Posture is a key factor, being seated or standing, each have a correct aspect to it. If standing, in case not yet tried, would suggest try a posture with one foot forward, and maybe open legs a little more, and bend the knees, but dont' be stiff, be bouncy. Your center of gravity to be between your feet (as close as possible), balanced. Your COG includes the instrument.
2. Also aim for the back to be as straight as possible, yet not stiff.
3. The best posture is not that of a statue.
4. The duration in which one engages into a give task. Often we concentrate on what we are doing that we forget or physical body and can over-strain it.
5. Learn and practice a little Yoga, in particular the Camel posture, and the Plow.
6. Can try meditation and its various Lotus-like postures too, for 10 minutes, or a little longer, on a gradual basis.
Best wishes!