A headache can be a sign of withdrawal, sort of like going without coffee for
a day? But if you get a queasy stomach and ringing ears along with it, that points to overdose. It may just be you're over-doing the
vaping, (sometimes we get carried away because it's new!) or your
juice may
still be a little high nicotine.
Maybe just slow up a bit, see if that helps. And take nice, loooooooong, gentle puffs. Draw into your mouth, then inhale. I don't recommend direct inhaling (pulling directly into your lungs) with vapor, that can be pretty stressful to your system at first.
You may also be puffing a little hard, I know my jaws hurt the first day. I was also flooding the atomizer too, by doing that. Try to relax with it a little more, I know it's hard to do that, especially if you're determined to stop cigarettes. It can be pretty panicky for awhile, until you feel comfortable. Part of that comfort is getting your nicotine level right for you.
If 6mg/ml juice seems ok, the next thing would be to just slow up your cadence a little. Drinking lots of water will help keep yourself properly hydrated ~ dehydration will also give you a headache!
Vaping has kind of a steep learning curve, for sure, learning all the causes/effects of something new, plus managing the basic equipment.
Best of luck to you, Welcome!