What Causes A Purple Bruise On The Right Shoulder Blade?

My daughter is turning 19 years old next month. She just sent me a text with a picture of her back. It is a big purplish roung spot that covers her right shoulder blade and a smaller one on the left shoulder blade. SHe is in good health. She has had a tight muscle down the left side of her shoulder that has been bothering her. What do you suggest? Thanks, Donna