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.
I don't know man. I wish you luck with it. Kitty's right one did that one morning several years ago. Now she's had both shoulders surgerized on. At least they are fixed now. Good luck with it.