I've had some similar incidents except on the left side. Turned out to be a muscle spasm. First time it happened I thought I had just slept awkwardly and tweaked my neck but it progressively got tighter over the following two days until I could barely move my head at all. Part of it was posture related and part of it was some old tennis injuries, doctor gave me a prescription for muscle relaxers and PT. Now I'm always trying to fit in a few stretches and exercises to keep it at bay. (The rhomboids and deeper muscles of the neck are the key for me) If it gets any tighter I'd go see the doctor immediately, the longer you wait the longer it takes for the muscles to relax again. (take a look in the mirror and draw on line on it to help see if you head is cocked slightly to one side, that's usually a sign) 

The pain is slowly going away. Also I have more motion now than before. I think I cut off my workout at the right time as to not aggravate it more. 

I did notice might right shoulder is slightly lower than the left. This could be the tight muscle pulling down on it. 

I've pulled muscles and ligaments before and there was not traumatic pain associated with this. My best guess is a pinched nerve is causing spasms and muscle tightness. I was getting some slight radial pain on my right shoulder. 

Right shoulder lower than the left is pretty common for any right handed person who has done golf or throwing sports. It's probably been that way for a while and you just hadn't thought about it.

There are a couple of things that you probably did... you either elevated your first rib.. this is often as a result of a heavy shrug type motion, or you created a "facet lock". The referral pattern of the lower cervical facet joints ( joints that guide your motion in the neck) goes into the mid trap area. 

Best way to treat these ( besides seeing a professional), is to lie on back and hourly do 20 gentle rotations to the (L) and (R), through the available pain free range. Holding the shoulder on the painful side down while turning also helps. 

I know we say RICE, but in this case it's not really an inflammatory issue but a mechanical dysfunction, so sometimes heat allows the muscles to relax and then you can re-establish the motion more quickly.