Liz, I has similar pain in my right arm for about 6 months and my doc was baffled. He agreed it was most likely muscualr and he advised me to get one of those strtchy exercise bands and to begin doing gental ballet type stretching. I found an excerise program for partially disabled folks that you could do in a chair I think I was called Sit and be Fit. The nurse who invented the regime appears on a local cable channel every morning and so I got to do her program for free. it really helped.

OK - found her website:
SIT AND BE FIT program

Don't be put off by the site saying it's for Seniors. My Doc recommended it because she shows 2 ways to do every exercise and one way is while seated in a chair. I still turn on her show a few times a week and follow along.

The other thing he did for me was to show my husband an accupressure point to push when the pain comes back. It helps to a degree but it does hurt worse at first.

Then, on my own, I put to use one of those hand and foot wax bath things you get as presents at christmas. If you take a paintbrush, heat up the medical grade parrafin and have someone paint the warm wax wherever it hurts and also along your shoulder, you might get some relief. The hot wax feels good and keeps the warmth on the painful area longer. I guess the warmth helps the muscle relax? When the wax hardens, just peel it off and continue applying until you get some relief.

There is another product that's great for keeping warmth on sore body parts but it's rather expensive. it's a spa product called Jelly Bath:
Jelly Bath
Click on products, then on Tub Soaks

It's stuff you add to a warm bath that turns it into a fluffy jell which retains heat for much longer than regular water. You sink into the warm jelled fluffy stuff and relax.   To get your shoulder soaked, you'd probably have to use two packets of the tub size which is waaaay expensive. I can justify buying the foot soak verisons for my feet but the tub soaks, I only do once in a while due to cost.