Caffeine before my workout makes me nauseous. Also high blood pressure occasionally. I suppose you could be hypoglycemic. What happens is that when you are putting your body under stress, your body's natural response is to shut down any process that isn't vital to your survival at that moment. That includes digestion. I can't usually eat for an hour at least after my workouts and even then the food tends to lay in my stomach.
The puking occurs because your body thinks it's in a survival situation and evacuates the contents of your gut. In really serious situations, your body will evacuate your bowels and bladder too.
Anything detrimental to survival is turned off... even pain. Think about it. If you took your shirt off and tried to walk through thorny brush, it would hurt like hell. But if you have a bear or a lion chasing your ass, you will cruise through that brush run for a mile and not feel shit. It's because pain, at that point, could prevent your survival. Soon as you calm down though, you're gonna feel it. Did you ever see those National Geographic vids where the lion is chasing the hare and the hare just barely escapes? What they don't tell you is that hare will usually get sick and die anyway because of the strain of escaping. Even your immune system is not necessary for you to survive at that moment. Which is also why you get the flu if you overtrain.
Your body is smarter than you are. It takes care of itself pretty well. All you have to do is take care of it and it will take care of you.
"He'll feel that in the morning."
"Scared me so bad I pissed myself."
"Trained so hard I puked up my lunch."
"Got into an accident and almost shit myself."
etc...
