There are many things that can do this and the answer is always: move to another brand of liquid first, then try mostly PG or mostly VG, to see if something can be identified that is causing the issue.
Avoid anything that looks as if it might have a bit of a kick to it. You may need a very 'mild' mix, with less flavoring.
It is impossible for everyone to tolerate all possible ingredients without some kind if issue showing up for some. Usually the problem will be minor, but it still needs fixing. It's likely that as many as 5 out of 100 may find one particular brand, or type of ingredient, may not agree with them. These are the most common causes:
- Intolerance to PG
- Intolerance to a certain type of VG
- Intolerance to a flavoring
- Intolerance to a low-quality PG or other ingredient
- Intolerance to a brand, that cannot be more accurately refined as to the exact cause
- A much milder 'build' of liquid could be needed - sometimes they are made on the strong side as people complain when the throat hit is too weak - and this doesn't help people who like it mild; less flavoring may help
With luck you find this out fairly soon - as in your case - and get the message to move on to another brand. I had a very strong reaction to one brand of VG and quickly moved on. No problems at all with other brands.
It's probably a good thing to find a problem early on. How to fix it several months down the line is a bit more worrying, as it would be hard to work out what is going on. If you get an unwanted reaction within a week then it's not hard to sort out.
With ultra-sensitive lungs, and unable to smoke more than about 6 or 8 cigs a day without getting bronchitis, my lungs haven't been better since I was young, after two and a half years' vaping.