What Causes Intermittent Shooting Pains In Shoulder?

Hi. Since waking up yesterday, I've had shooting pains that feel maybe a little bit larger than a pin-prick in my left shoulder blade. It lasts a few seconds and then fades away, and there's a couple second wait between them. Yesterday, it took several hours to wear off, and now they're back this morning. It hurts to the point where I semi-involuntarily shrug my shoulder really hard just to get it to stop.