It's the irritation in the lungs from smoking--any moisture, even steam from a warm drink like coffee, will cause the coughing fits. I was that way when I first started vaping, too. I was truly surprised at how quickly I would go into a meltdown after inhaling just the slightest bit of vapor. But I knew that it happened with warm drinks, too, not just vaping, and I also remembered that it didn't happen when I was younger. So, I worked on cutting out the smokes. It was only after about a week of zero cigarettes, that I could tolerate vaping without nearly breaking a rib. To this day, however, I cannot tolerate very warm vapor without coughing.