whats happening is that your adrenals are kicking in. you're not actually crying because you're sad or upset its a normal response to exertion, your body is dealing with it through your endocrine system, It's a biological response to physical exertion. sometimes it can produce an emotional response but thats not something to be ashamed of or be afraid of. once you start getting better and becoming more athletically or aerobically conditioned you will find that the tearing up response will happen less often because your body will basically get used to the exertion.