What Causes Severe Chest Pain On Left Radiating To The Left Arm?

I woke up this morning with severe left chest pain, radiating into my left underarm. I am 60 years old and in poor health. The pain is so bad, I can't take a deep breath. Sitting, rather than lying down seems to help ease the pain. I am just getting over being sick (since 11/14/14) with upper respiratory distress and exacerbated asthma. Is this muscle pain from coughing, or something more.