I am having sharp pains under my left shoulder blade when I take a normal to deep breath. The pain sometimes shoots down my back. What should I do?

Should see your PCP- For an examination, chest xrays, labs, maybe a ct scan, depending on what the pcp finds. Possibilities include rib fx, infection, pul embolus, strained muscles; as all these result in increasing pain with deep breathing. A spine cause will be diagnosed with ct. See him sooner than later, to avoid any serious problems with delaying to see him.