It must be going around. My left shoulder grinds and pops when I rotate my arm. No injury, I just woke up that way one morning. I am slide sleeper. I'll let my Chiro work on it. She'll get it back into shape.
One morning last week, when I woke up my left shoulder was sore and down into the side of my ribs. I rubbed in some sports cream. As I was doing rib area I felt a little 'crunch' And 3/4 of the pain was gone almost instantly.