If a vendor cannot tell you EXACTLY what is in the liquid, find another vendor. If you are having medical problems, they should be bending over backwards to help you out.
The vaping community is a pretty friendly bunch for the most part. Everyone I've dealt with in the industry has been extremely helpful.
Some small shops buy their liquid from someone else. Most good vendors make their own. (try Mt Baker Vapor, they are inexpensive and pretty damn good)
Some of the juice makers will add a variety of things to make their juice unique. There are throat-hit enhancers, flavor enhancers, sweeteners, etc.
You could very well be allergic to one of those items as well.
You can always go the DIY route to make sure you know what you are getting (and save a crapton of money in the process).
Wizard labs sells the PG and VG base in whatever strength of nicotine you want. Mix in your ratio with the flavoring of your choice.