I think my quest to quit smoking has led me to a stronger addiction to nicotine than I had before when I was smoking a pack of marb lights a day 4 months ago. I no longer smoke but I vape all day long like crazy n I always get 18 mg or 1.8% from johnson creek. I no longer crave analogs but my vaping I think has gone out of control if I don't have it for a short period of time I get very mad and frustrated. Thing is I have no idea how much nicotine I am getting I vape about 2-3 ml a day of Johnson creek domestic 1.8%. My job is very boring and I sit on a chair with my feet up for 9+ hours out of my 12 hour shift so I just vape and vape and vape... Sometimes I feel like I overdosed on nicotine as I get a headache n feel sick to my stomach. I feel like if I bump down the nicotine level I will be cranky n mad all the time.
Sounds to me like there are a number of factors at work with you: First off, be aware that it can take any number of months for your body to completely adjust to being off cigarettes (it depends on your particularly physiology and how long you smoked). Some of what you're feeling may be quit-smoking effects and little to do with vaping. Also, both headaches and nausea are early warning signs of dehydration, as others have said, so do be sure to drink enough water. Don't wait to feel thirsty before drinking.
Also, your chain vaping may be related to physical rituals (hand-to-mouth, exhaling vapor) and opportunity more than to "addiction" or "dependence" on nicotine. As a smoker, I had to have that cigarette at regular intervals, regardless of what I was doing. It often meant stopping in the middle of a task to smoke. As a vaper, if I'm at the computer or reading or watching TV, I'll chain vape, simply because I can, but when I'm doing something like washing the car or mowing the lawn or painting a wall, I can go hours without even thinking about having a vape.
Regarding your nic levels, use what works for you, but I'd suggest you try keeping your devices loaded with a variety of levels and using them in rotation. This wil allow you to go to a lower level at times but you'll still have the 18mg available if you feel the need.
Just remember that there are many more factors involved with the "habit" of vaping than the nicotine itself, and that nicotine has been shown to be as addictive, or dependence-forming, as the caffeine in your coffee or soda.