Nicotine - try 18mg. I smoked 1.5-2 PAD for almost 40 years with the last decade on Lights and 18mg was just right for me - give it a try.
The most typical symptom of too much nic in too short a time is headache (and never in my life did I get headaches until I started vaping, and when I vape too much in a short period I get the headache). Other common symptoms of too much nic - rapid or irregular heartbeat, cold fingers, hazy or ill feeling.
Some other headache possibilities
Dehydration - drink plenty of fluid, vaping is drying. And I recommend e-lquids with some VG in them (I use PG w/ 10-20% VG) - a bit less drying and buffers the irritation of nicotine while adding a bit of nice vapor
The larger size of the mouthpiece vs cigs (especially for TMJ sufferers) - try a drip tip as a mouthpiece on cartomizers (I recommend Boge as first cartomizers, the most popular cartomizers on 510 threaded devices)
Biting on the hard end
Unnatural position when vaping, stressing neck muscles etc
Dragging too hard - not needed. Press the button in 1-2 seconds early, start dragging slowly, let the button up just before the end of your drag, finish drag, inhale
Atomizer + cartridge = Juice in your mouth sometimes = more nicotine absorbed. try cartomizers
Stress - buy spares of everything, have spare charging cables at work and home, I like to leave a spare set up overnight.
I have since worked my nicotine down to a low level and finally I can vape a lot at times and not get a headache. I also moderate my vaping by leaving the ecigs in a nearby room at home, and only put one next to me if I know I'll be stuck away from that from for 20-30 minutes or more.