Since I don't have access to a gym, the last few days I've started running once every day or two. My cardio-vascular system is out of shape so I can't go for more than about 30 minutes with breaks walking in between, but I've found that what's stopping me from going for longer is that when I start breathing in more deeply, my chest hurts. It feels like it's on my ribs, and its a dull pain that becomes sharper as I breath harder and go for longer. Does anyone know what I'm talking about, what causes it, how to fix it, etc?