The most likely cause of this pain is actually just a strain or sprain in the muscles that run between your spinal column and your shoulder blade. This kind of injury
is very common from physical activity or lifting heavy objects, but it can also occur just from "sleeping funny" at night. Generally speaking, the injury is not something to worry
about - it gets better on its own slowly over a period of days to a week or so, and you can help things along by taking an over the counter anti inflammatory medication like ibuprofen.
If, on the other hand, you had a recently accident involving the shoulder, or if the pain is worse when you lift your shoulder up over your head, then that would be more consistent with damage to the muscles or tendons in the shoulder itself. The most common kind of shoulder injury is a rotator cuff tendonitis, which is an overuse injury caused by repetitive motion at the shoulder. If you think you might have a shoulder injury you should go to see your primary care doctor
who will be able to examine your shoulder, confirm the diagnosis and rule out more serious injury, and discuss treatment options.