IMO as a nurse and a vaper for over 2 1/2 years, I believe the PG is a humectant, this means it is a substance used in many foods and beauty items to moisten them. With this in mind, view the vapor as a moistening agent to the lungs. Our lungs and bronchials are covered with tiny hair-like structures called cilia. When smoking for a period of time, these little hairs become gummed up and no longer filter the air we breathe as they should. In addition, the function of our pulmonary tissue is not as elastic, so inhaling a large volume of vapor when there is not enough room for it will cause the nerve in our trachea to reflex a cough. When I began vaping, the cough was tight and irritating, and I did notice some flavorings were more cough producing than others. After about three months the cough loosened and I was expectorating copious amounts of grey mucous. My pulmonary function tests are now normal, I have no cough and medically speaking, according to my CBC, blood gases and my chest sounds and pulmonary function, I am clinically considered a non-smoker. This is all true despite the fact that I chain vape all day at very low nic and use 12 mg nic in morning and night for increased craving times.
Hope this helps.
Keep on vaping, it will get better.
PS....IMO I do not believe it is related to an allergy, possibly a sensitivity to a flavor but not a full blown allergy to PG. PG is found is so many products it is rare that one would have a true allergy to it after all of the exposure since birth. It is possible, though, but rare IMO.