If it is broken, gently probe it to find out where. If you can't feel a break, it probably isn't, although coughing without pain probably means it isn't. I doubt it's lung related if coughing doesn't hurt. Diaphragm maybe? Could be a blood clot, but I doubt that, because you're young. Could just be a bruise where you've slept on it. It's not an air bubble, because you're still alive.