A Mystery for You

Okay, I need some help here. I need someone to explain this to me because I am completely and totally stumped.
I just spent 3 hours in the emergency room with my 12 year old son. When I got home from work today he was home and on the computer playing WOW. No problem there, this is pretty normal but he was moaning and complaining that his ribs hurt.
Well I really didn't think too much of it because he was also eating two HUGE sandwiches and I just thought he was whiny cause Mad Hatter was making him get off the computer so he could play.
So Caterpillar goes into the kitchen and by this time he has tears in his eyes and is being very short with us, so I went in there to adjust his attitude and he is bent over. Mad Hatter came in and had him pull up his shirt and an area in the front of his rib cage is swollen. Noticeably so.
So now, as I am collecting my Mother of the Year award, I am on the phone trying to find out if possibly the doctors office is still open and I can get an appointment. No go, but they told me to take him to the ER to make sure.
So off we go to the ER.
On the way there Mad Hatter and I are quizzing him to determine what could have happened. The only thing that he can think of is today at school he fell asleep while watching a movie. His friend hit him to wake him up but I just cannot for the life of me imagine that he hit him hard enough to hurt him like that.
Xrays came back and they believe his ribs are cracked, which of course they could do nothing about except give him pain killers. So right now he is in his room, playing Xbox, while in happy Vicodin land.
Now, yesterday his sister totalled her car and walked away from it without any injuries, just some stiffness and aches. Today, this child who was nowhere near the accident,cracks his damn ribs without any noticeable provocation. I swear, the whole time I was in the ER I kept waiting for CPS to show up and accuse me of beating him in his sleep.
I just don't get it. How in the world could he have cracked his ribs and not actually remember HOW it happened?????? Is it possible that his friend actually hit him hard enough to crack them????? Is this one of those freak accidents???? Or did I really go in there and beat him in MY sleep?????