I would have guessed maybe a severe spasm as I have had the same thing happen to me, except it was only my left shoulder, neck & upper back. I also felt like something was pushing forward with lots of force into my upper left side/shoulder. I had some problems lifting my arm in certain directions and when I layed down, my left shoulder would not lay flat at all, no matter what I did. It hurt to lay on that side but it felt better than laying on my right side. This was because while laying on my right, my left shoulder area then felt like it was hanging forward too much and was painful in pretty much any position. I couldnt even do my crunches correctly because it would spasm more. LOL, i even had someone I know pull on my left arm because it felt like it was out of place or something (however that did not help any).
At the the time, though...I also had severe lumbar stenosis where I couldnt realy walk anymore & the little I could, I had to hunch over in almost a 90 degree angle. SO I just figured all my upper neck/shoulder/arm pain and spasms were because I walked funny for so long. After I had surgery it pretty much went away for me (though I am noticing I'm still getting lots of lighter, less severe cramps & spasms all over).
I am happy though, that whatever it was for you has finally resolved itself!