I dont know if anyone answered his original question,
But there r only two other things i personally can think of, or maybe i didnt read xlose enough and u already figured it out
1. the spitback is hitting u so hot it is burning your tungue, same as a hot cup of coffee, or maybe just the whole vape cloud...your mouth and lungs will all be desensitized from smoking, so u might not notice 'too hot'
Or maybe it is an actual for real allergy
might be worth stopping in real quick just to let a doc give u his opinion just so u know what not to do in the future.
those 2 issues, as well as others) wont necessarily be resolved by switching equipment or even juice if u dont know what in the juice is causing it, and for some people, vaping is simply not an option unfortunately