If this is a pinched nerve, then it means your muscles have not settled exactly where they should sit naturally.
The best thing to do is to put a jacket on and go for a run, start to use your muscles so that they contract and retract, heat up and start to relax. A bath will maybe help temporarily but you're not using your muscles, so you'll probably end up just loosening a knot that'll tighten right back up again when your temperature drops back to normal out of the bath.
Massage the back of the shoulder, around the top of the spine, most nerve issues up top originate from the spine, when your central or peripheral nervous system is slightly misaligned.
Rowing motions, (Seated rows if you've got a gym) require a lot of upper back muscles to squeeze and move, so I'd recommend that.
If it's a pinched nerve, then it's not an injury, so just blast out some exercise until your body naturally restores order.
If it's a torn rotator cuff, then you'll probably try to do a seated row and end up crying like a girl and running home, but at least you'll know