Thank you for your query at DoctorSpring.com
It seems like you have strained your muscle and that is causing the pain.
Did you sleep weird before too? Because a lot of people have a habit of sleeping prone, that is laying on the top of the arms. That puts a lot of strain on the shoulder joint. In fact it becomes a chronic pain and it may take around a month or two to heal.
I need to know what is your preferred sleeping position?
If you can avoid sleeping in any other position apart from supine ( laying down on your back ), for the next few days the pain should subside. I don't want you to start any painkiller unnecessarily, if the pain automatically corrects itself after you change your sleeping posture. Meanwhile you can use a muscle relaxant spray to get relief.
I am not concerned about other causes since there is no history of trauma or injury. It's also reducing on its own, so the posture seems most likely cause. This kind of pain mostly is felt all around the affected shoulder joint, underarm and back.
If it settles down on its own after using muslce relaxant within a day or two then no other intervention will be required.
If it doesn't settle in 2 days then you can take Tab. Myospas Forte BD twice a day, after food for 5 days. If it persists beyond a week then an x ray will be required.