What Causes Shoulder Pain And Back Pain With Breathing?

I have been having these off and on pains (happens about 3 times a week) in my upper left shoulder and my lower center of my back ... but only when I breath. When I stop breathing the pain dulls down but as soon as I take a breath the pain flares back up. I'm only 19 and I have never had any health problems before now. How come this happens?