What Causes Sharp Pain In Shoulder Blade?

For approx. 1 week I ve been getting sharp pain in my left Shoulderblade. It s extremely painful at times. I m wondering what would cause this? Could I have a pinched nerve or something similar? it s not chronic pain, but rather comes on when I move a particular way. When it does flare up however it s exceptionally painful and feels like needles in my Shoulderblade.