The closest I ever came to throwing up from a training session was years back, when I took a private Muay Thai lesson.
The instructor had me spar with him for five, five-minute rounds the very first time out, and I felt like I was going to be sick by the end of it.
Mind you, I didn't "hold my own", the guy could have kicked my ass readily, I was staggering around the ring.
I think the first lesson was more an assessment of mental toughness than anything else (at least that's what I tell myself)
In training, I've had clients get sick more than once.
While I do my best to avoid this happening, some people are highly sensitive to it, and it's also largely GENETIC as well.
Some people can be very de-conditioned, and they never get sick.
And someone can be an elite/professional level athlete, and they get nauseous with regularity
So what's the reason WHY?
1. Inappropriate nutrition
Ever wonder why your muscles are not "pumped" looking 24/7?
The reason for this is because most of your blood circulates around your INTESTINES. Meaning your stomach.
When you start lifting weights, that blood moves from the stomach to the muscles
If you have a lot of food in your gut, that food then sits there, indigested. And this does not feel good.
This is why "peri workout nutrition" is a concept worth learning.
When you combine heavy food with sweating (which dehydrates you), rehydrating with plain water (which has no electrolytes), and then rising aciditiy levels in the blood (from the exercise)
The effect is a combo of "Im sick from the food, Im dehydrated from the exercise, there is too much water in the stomach, and my blood is acidic. GET RID OF SOMETHING"
And the result is that you Vomit. So that's the first reason.
2. Dehydration plus Overexertion
Let's say you are out of shape.
And let's say you decide you are going to do something really hard, like a fat loss workout you've never done before that's 20 minutes long and you are doing endless burpees.
And let's also assume you are like most people, who apparently believe that drinking water isn't important, and all you drink is coffee and rockstars.
So you do this workout, and you get horribly sick from it, even though you haven't eaten anything, what gives?
-Low electrolyte levels
-rising levels of blood acidity
=Body in panic mode=THROW UP AND STOP THIS
The above can be solved for with proper hydration levels, consuming electrolytes, and keeping your exercise "reasonable" and not ridiculous.
3. Your workout was too hard
-This is the humbling one, and the one that leaves people realizing "Ive really let myself go"
The workouts that tend to make people sick are total body workouts.
The body is completely unaccustomed to the change in blood flow, your bodies recovery mechanisms are basically nonexistent, and the entire experience is interpreted as a traumatic event.
Ive had people get sick from doing lunges, from squats, from pushups and rows.
Often the exercise has been NOTHING that would even be considered intense, the individual is simply so de-conditioned that anything that even raises heart feels like its going to kill them.
The exercises that often cause this to happen are ones that blood flow rushes either to the lower body, or to the head.
Movements done on the floor to standing are infamous for causing this (along with dizziness).
If this is you, you've got your work cut out for you, this is going to be a lengthy process.
I would advise hiring a trainer in this situation, simply to keep yourself from hurting yourself if nothing else.
So all these things said
-Hydrate with fluid AND electrolytes
-Get an expert if you're hopelessly lost.