Nic sick, almost certainly. Maybe a bit of vaping dehydration, too. Cut your nic level to something like 6mg and drink more water. Give it a day or so to start making a difference... your body needs to process that excess nicotine for you to start feeling better. If you don't mix your own juice, go buy a new bottle of the same flavor with 0 nic. Then mix them evenly to get 8mg, or use more 0mg to get a lower dose. Even at levels as low as 6mg, nic sickness can happen, especially at high wattage settings. I personally vape zero nic in all of my flavors with one expection where I vape 3mg. Even with 3mg, I can really feel it if I drip or sub ohm. Technically speaking, nicotine is a toxin, but is safe and fairly asymptomatic at lower doses. If you introduce a 16mg juice into your body at high wattage, you're litteraly poisoning yourself slightly... those symptoms you have is your body's way of telling you to stop ingesting that much nicotine.