I think those of us who have asthma or other smoke induced respiratory problems are going to be far more sensitive to anything we put in our lungs. Vaping pg/vg, nicotine and flavoring all have some form of irritation to the lungs...and anything that irritates the lungs leads to inflammation and inflammation leads to shortness of breath. I think for many these irritants are minimal, but for others they create havoc on their lungs...probably due to already having existing respiratory problems or asthmatic allergies.
Pg can make the lungs feel dry and raw given it has a drying effect. Vg is like a sticky heavy coating that makes the lungs feel heavy with discomfort. Then there is also the added irritation of nicotine and flavoring. All 4 of these are irritants working at once and when one or more of these ingredients poses a heightened irritation to your lungs, it causes any minor irritation by the other ingredients to be amplified..which spins into an inflammatory cycle that is now compounded by 4 different ingredients agitating the lungs in different ways for different reasons.
Now all of these things are pretty harmless given what we know up to this point with the exception of flavors, but you must also remember that it is not uncommon to see people who either can not tolerate one or more of these ingredients, or can't tolerate certain levels of it without negative side effects/lung irritation. So while these ingredients are thought to be pretty much harmless again keep in mind, anytime you put something into your lungs, it's not a question of IF it irritates the lungs, it's how badly will it irritate YOUR lungs that determines the final verdict on whether it's harmless or not for YOU.
If you find that no matter what combination you use or ingredient you take out you still have this problem...vape less, or try to get into the habit of inhaling less often when you chain vape. Mix up mouth to lung with mouth to nose so you are not constantly inhaling it into your lungs. It's very possible your lungs may start becoming overly sensitive to the ingredients only when you vape too much to frequently. I am sure once you start taking apart your liquid and experimenting with the ingredients one by one you will easily target the ones that trigger shortness of breath. That's the beauty of vaping..we can add or remove things that help or hurt us on a individual basis..unlike cigs.