I have never experienced nausea or vomiting during, or while having an enema. I have had nausea, and subsequent vomiting, after having used multiple enemas. In one instance, I had cleansed prior to a medical procedure. I was NPO until the time of the procedure, and had taken 2-3 enemas beforehand. After getting home, I took another large tap water enema for the fun of it. I was expelling and began feeling queasy. Shortly thereafter, I hurled. Recognizing the error of my ways, I hurriedly replenished my electrolytes with liquids, vitamins, and a couple of bananas.
I knew of the potential risks, but was too worked up to ignore the urge at that time. It was a mistake that I only made one other time over my 60 years of indulgence in my favorite pastime.
Multiple enemas/colonics can cause some electrolyte imbalances. It may relate to sodium levels, magnesium levels, dehydration, or water intoxication among other things. That is why we need to be cognizant of the issues and always be safe and sane with the solutions and quantities.
While there may be no verifiable health or medical benefit to our indulgences, the psychological rest, relaxation, stimulation, etc., is sure a source of some pretty simple pleasures. Enjoy a good enema for some shits and giggles, just don't overindulge to the point of harm!