Just Woke Up With It / Just Started Hurting

Usually, when you notice the pain 'just come on' without any specific injury, it is something that has been 'brewing' for a while. In this case, it could be something as simple as a small adhesion that was formed 10-20 years ago that has just gotten bigger over the years. Or, it could be something much more invasive, in which case you would need to get the shoulder checked by a doctor immediately.