CandP, I've got exactly the same problem right now. I"m assuming that I slept on it wrong, and I've krinked something. The top of my shoulder at the back is just aching, and it feels weird down my arm. Then, my whole arm feels very heavy and sore. Then it eases up for awhile. Then the whole thing starts over again. I"ve tried stretching it out, but that really doesn't seem to be helping at all.
When I go home, I'm going to get an icepack and put it on the back of my shoulder. Maybe it'll help.
Let me know how you're doing with this.