She needs an exam and a better history. BigGulps is right that the two most common things that would do this is early Frozen Shoulder or bursitis. There are some more rare conditions that could do this that include Brachial Neuritis/Parsonage Turner, gout, AVN, tenosynovitis or radiculopathy from the neck. It's also possible, but unlikely that a rotator cuff tear decided to get super pissed off without any injury.

She just needs a good ortho/sports med doc to see her. Good luck to her. I hope she starts feeling better soon.