Go to the doctor. I had a muscle spasm about 8 months ago...I was camping on a trip to go white water rafting, I laid down to go to sleep and my neck just felt like I broke it. I ended up sitting outside the tent stretching for 6 hours instead of sleeping and loosened it up. Turns out, that was a bad idea...that pain was a virus attacking my nerve causing the muscles to contract. The virus "stunned" the nerve because I was trying so hard to push through the pain. With the nerve stunned, the muscle holding my shoulder blade in quickly lost size which caused my shoulder blade to push out backwards anytime I moved my arm above eye level. By the time I got to a doctor and got an mri, I had a torn rotator cuff along with the "stunned" nerve and a muscle that had shrunk to about 1/4 normal size. I am still doing physical therapy and still don't have full use of my arm now after surgery.
Moral of the story...go to a doctor.