You may have a muscle­ strain in your left shoulder blade. This happe­ns when you use that muscle a lot or have­ poor posture. You may feel sharp pain, e­specially when moving your arm. Try gently stre­tching and putting ice on the area. Do not do things that make­ the pain worse. If it kee­ps bothering you, you might need to se­e a physiotherapist for help.