It could be anything. Have you taken a bad fall recently? Have you fractured or badly hurt your clavicles or scapulae before? How long have you had this discomfort? You should probably see a chiropractor as we cannot give you an accurate diagnosis over the internet.
More information might help here. If the pain came out of no where, could be muscle pull. However if it is kind of continuous pain, then rheumatic could also cause the pain. Seeing a doctor will be the wisest way to find out.
Looks like it's time to see a Chiropractor. Usually you're not supposed to redirect answers to other sources, but it could be a very numerous amount of things, such as the way you sleep, a growth defect, etc.
Yaa a fall in 1st grade hahah but no never injured my back For like 3 days i think i pulled a muescle But idk