Could be a number of things. Not enough food or hydration like has been said already is possible. I've heard of people getting sick because they ate too much close to a workout but you said you did the opposite so it isn't that. Sounds more like you body wasn't prepared for how hard you got pushed and it tried to make you back off. The body can only handle so much stress before it starts to break down and tell you "Hey! Back off!" Just make sure you eat and drink enough before a workout and get used to pushing yourself a little harder each workout and your body will adapt to the stress and be less likely to crash on you.
i was getting crook a while back also,
made sure i ate 2 hrs prior, removed this low fat margarine i was having on whole grain toast and it stopped,
now im on oats with a diet yogurt once they cool, banana, yakult
1 cup water 1 hr out, 1 more 30 mins out, 300mgs caffeine walking out the door to the gym,
10 minute walk there, im warm and pee before i start, never been sick again
Nothing is so common as the wish to be remarkable.... ~ Shakespeare