Here is whats happening. You are dehydrated. This is giving you headache and nausea. Drink more water when you vape. This should help i think. I get it too sometimes when im vaping a lot and not drinking water. Pop or beer or anything like that won't help much.
This!^ Its either because you don't drink enough or because you have too much nicotine.
Since vaping causes to suck moisture out of your lungs you really need to drink water. an average human drinks anywhere between 3 to 4 liters of liquid a day for good hydration.
When vaping it should be a little more
It also could be that you are just administrating too much nicotine into your body at once.
So you could indeed drop the 6mg to 3.
I started out with a eleaf basic and a 2ml tank with 0.75ohm coils. I smoked liquids with 18mg. Since it was new and the tastes of eliquids are rather good compared to analog cigs ... I vaped like an insane mad man.
I got nauseated , got headaches , dry mouth, dry/sore throat. Then I started to DIY.
I mixed my own liquids and made lower dosages of my liquids. I went from 18mg to 12 to 6 to 3. in matter of months ( November 2015 to June 2016)
I bought a new regulated boxmod in July (SMOK XCUBE II) with a Geekvape Griffin 6ml tank. At this moment I was still smoking 3mg liquids. Ialso started to vape with lesser resistant coil setup (0.22 ohm) creating a lot of vapor but also draining my tank very quickly.
And guess what... I got nauseated, headaches and all that stuff again. (too much nicotine in a short period) so I dropped to 1mg...
I have no problems now anymore.