Hello Doctor, I was at the gym last night when I started feeling a pain in my back. I thought I may have pulled the muscle, but then the pain felt like it was coming from within, and spreading to my chest/left side of upper body. It would hurt whenever I breathed in. When I went home, I laid down and the pain became excruciating. All of a sudden, when I get up, the pain is no longer in my chest but in my neck/collar bone. My lung on my left side has partially collapsed before. What could it be?