I just wanted some advice before I visit a doc - I woke up from a short nap, lying on my back as always, a couple of days ago, and noticed a pretty intense pain in my shoulder as I got up. It's just as bad today - doesn't hurt when I lay motionless but hurts lots if I let my arm hang relaxed and if I move it. Seems odd that such a pain - from the base of the neck, across the the top of the shoulder - should come on so suddenly during sleep in a regular position.
Sounds like it may be the trapezius muscle, a large kite shaped muscle across the back of the shoulders, from the base of the skull to the lower back. It helps hold the upper extremities up. It holds the shoulder up when you sling a backpack over just one shoulder. It also helps when carrying of suitcase or bucket. It's not uncommon for muscles to cramp or spasm with minimal ischemia. Just lying on it wrong can sometimes bring it on. If the discomfort doesn't resolve, gets worse, or has associated symptoms (numbness, tingling, weakness, etc), you might consider seeing someone about it. In the mean time you can try symptomatic treatment such as; ice (for acute pain/swelling), warmth (more soothing for spasms), hot tub, massage, or OTC medicines. Hope it feels better. Good luck