I second the welcome to ECF!
Stosh is absolutely right; after a month of not smoking, it's very likely just a side-effect of cigarette withdrawal, but it could very easily be from dehydration, which can give you a headache.
For myself, I get headaches from not enough nic; too much just makes me nauseous and gives me a stomach-ache, but it could be the other way, too, from too much. 18mg is pretty high, so if you're not having massive cravings, maybe go down just a bit, and see if that helps.
You didn't mention your PG/VG proportion; once when I vaped much too high a level of PG, it really blistered the inside of my sinuses, and my throat, so if you're using a lot of PG, it could be something along those lines too; everyone seems to have a different threshold of tolerance for PG, from can't take it at all, to those like me who usually vape 80%-85% on a daily basis -- but if I go to 90%, I know now to not do it for more than one day, or risk that blistering irritation again. But irritated sinuses obviously can cause a bad headache.
Welcome, and hang tough!
Andria