What Causes Pain In Shoulder And Arm When Taking Deep Breath?

I am having pain in my left shoulder and arm. Its a dull ache, like I slept on it wrong. Except that it hurts when I take a deep breath, yawn, laugh, or bend over. I kindof have that pain in my left shoulder blade area too. What could it be? And when should I go see a doctor? I am female. 36 years old.