I vape 3mg, and too much of that makes me dizzy... Although I do cabin vape on a dripper a LOT, if you're vaping 18mg I can see where your problem is! I also have reactions to PG that I won't get too graphic about, but let's just say a gurgling stomach and lots of trips to the bathroom!
I only use high or max VG and have no issues what so ever now.
Everyone has different reactions to ingredients and nicotine, so it could be any one of those.
I would suggest sticking to well know brands of juice (all mine are the big US brands like KILO, cutwood, vaping rabbit, VIGILANTE etc who release toxicology reports of their juices, show their clean rooms where it is made etc etc).