I’ve been going through upper back and shoulder problems for a few years now. There are many causes to it, and you need to be careful because you don’t want to cause any long term injuries. If your pain persists, you should probably seek the advise from an Alexander technique teacher, massage therapist, or physio therapist. Personally I find I can play for longer without being sore when I’m standing versus sitting. I don’t know why that is though.
Here are a few causes that I know of:
-Even when thinking about posture, violin tends to make people roll in their shoulders, which makes everything go out of line.
-Since one side is holding still and the other side doing many repetitive motions, it is easy to become unbalanced between the right and left.
-When we are incredibly focused on our playing, it is easy to not notice if we are being too tense.
-If your chin rest and shoulder rest are not the best match for your body, it can cause the shoulders and neck to do bad things in order to compensate. People with short necks usually don’t have much problems with this. Although, people with long necks like me often needs to experiment with their setup. First find a tall chin rest so that your neck doesn’t have far to go. That way it puts minimal tension on the back of your neck. Then find a shoulder rest to match.
-Remember to stretch your arms/shoulders before and after you practice. Think of it like going for a jog, where you need to do a proper warm up and cool down in order to prevent injury!