Nothing caused it. It was just spontaneous. Quite common I guess for guys of my stature and age. 18-25 tall and skinny. Although I'm not very tall.
It feels like you can't breathe, lol. The left side of my chest just hurt like someone was driving their fist into it. Not punching, but just trying to break through the flesh or something. If I didn't breathe deep, it wasn't that bad. But it still sucked, heh.