I see it too often. Non-specific shoulder pain that limits a person's function and overall quality of life.
The good news? It's rarely serious and usually gets better.
The bad news? People don't know where to start, let alone what to do.
There are lots of causes of shoulder pain. Huge deficits in internal rotation range of motion are also common. Instead of worrying about "naming the tune", you can try this quick self-management technique (Video) and see if either pain or mobility get better.
**SPOILER ALERT** Two minutes spent on the above technique usually helps both, so get after it!