Andria's advice is good, try vaping pure Glycerine, USP with around 10% distilled water to thin it. And make sure to get it from a different vendor than your current supplier. If that works out OK, then add 5% PG and see what happens. Some PG is a good idea but too much, in rare cases (and your experience *is* rare) can cause problems for some. Then if all is still OK, get some 100mg strength nicotine in VG and add a little of that. Finally, if all is still well, add a simple flavor.
Using this process of elimination / re-introduction should help locate where the problem is.
It is difficult to describe vaping as harmful to the lungs, in and of itself. If moderation is employed, and safe materials used, then all it is likely to do is help protect from viral infections like colds and flu - since aerosolised PG is one of the most powerful bactericides and virucides known. People have been inhaling it in asthma inhalers for decades, with no implications apart from the occasional minor intolerance to large amounts of PG in the upper airway, as is usual. On the plus side, nicotine is about the best safe cognitive function enhancer we know of, and helps protect against certain medical conditions.
Problems may come with high volume use, excessive inclusions and consumption of any one of the refill ingredients, specific intolerances, and specific allergies. The most likely areas that will produce future problems are high volume consumption, super-hot atomisers affecting the vapour structure, and flavours that turn out to be unsuitable for inhalation.
