I should elaborate on this. I had what is called spontaneous pneumothorax, which means my lung "popped a hole in itself" without any external trauma. Unless sitting in a couch can be considered trauma...
Anyways, walking home that day was like walking with a twisting knife blade in my back. Holy crap it hurt. But since I am so bloody naïve, I shrugged and thought "meh, it will pass". Sure enough, it got better. Can't remember exactly how things developed, but I went to the doctor and he thought it was an infection, so he gave me some anti-inflammatory pills and sent me home. Three weeks later I returned, and he instead sent me to the x-rays.
"Whoops, son, you've got a hole in yer lung! That's not good."
So I get sent to the ER, where they look at my x-rays and say,
"Well, not much we can do now, come back in a few days and we'll take another look."
Seems it was healing itself, so I only had to return a few times for check-ups.
Long story short, it is a condition that tall, skinny males of that age can contract. I was lucky they didn't have to stick a tube in my chest and vacuum out the air pocket.
Oh, human body, you so random.