Unprovoked intense shoulder pain

Bramish,
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