I did a search and found this
Getting sick after a workout can be caused by several different things;
2. Heat exhaustion
3. Vagal reaction
5. The most Likely Causes of Exercise induced sickness.
1+2. Exercise induced sickness from dehydration and heat exaustion is usually because of the increased effort causing you to sweat out lots of fluids that are not replaced.
3. Vagal Reaction - Exercise induced sickness from the vagal reaction is more rare but happens. Most folks heart rate increases when they are excited, scared, or doing a hard workout. However, a small percent of people experiance the opposite. Thier heart rate slows, blood pressure
falls, and they can experience intense nausea with or without vomiting or getting sick.
Here is a quote from National Library Of Medicine
"Vagal reactions" may follow sudden emotional or physical strain in "vagotonic individuals" and are usually benign. In the case described, emotional stimuli caused life-threatening vagal reactions in a "healthy individual.
Implantation of an artifical pacemaker was necessary to control the attacks. HMMMMM, sounds scary!
4. Exercise induced sickness from Hyponatremia is caused by a sudden drop in concentration of sodium in the blood. When you do a hard workout with heavy exercise and sweating, your body turns on the pituitary gland in your brain to produce a hormone called ADH, or anti-diuretic hormone. This hormone tells the kidney to hold on to free water or water that does not contain salt. If your salt and other mineral intake is too low, then the concentration of sodium in your blood can drop. For some reason people that take anti-inflammatory medications like Motrin, Advil, or Aleve may be more at risk of hyponatremia.
5. Gastrophageal reflux brought on by the valsalva maneuver or holding your breath while you push hard. BREATH SO YOU DONT PUKE!
A weak esophageal sphincter or a looseness of the little ring of muscle that separates your stomach from the esophagus. This is more of a little regurgitation instead of a full fledged puke.
If you are shaking when you puke then is could also overstimulated your central nervous system
from intensity techinuques or just going past your limits and your body wants you to stop! So it makes you puke!
The last thing and what my intuition tells me is the most common cause of exercise induced sickness or the puke monster is a change in your blood pH produced by the anaerobic metabolism of pyruvic acid. If you workout harder than usuall and your body cant get rid of the lactic acid quicker than you are producing it a build up occurs and your ph goes out of whack. You become toxic and puke to try to clean your system.
So why do you puke after you work out? In most cases the bottom line is that you worked harder than you should have and your body wants you to stop. Making you puke seems to achieve just that for most people!