I find myself constantly
vaping too. Can you overdose on the nicotine?
You betcha... head spins, rapid heartbeat, jitters (shaking hands, nervousness), stomach pains (sharp like gas pains all over abdomen), nausea/vomiting (that's the worst one!), ......... Headache is generally more a sign of not enough nicotine, rather than too much. Since I know I'm vaping a lot less nicotine than I was smoking, I sometimes get a nagging headache, but if I really vape like crazy for a few mins, it generally goes away all by itself. My earliest sign of too much is the sharp stomach pains, feels like gas pains, the little knives poking you all over your abdominal area; if I don't quit vaping for a few mins and then switch to a lower level nic, it's followed pretty quick by nausea. My earliest sign of too little is the nagging headache. Not a monster headache like a migraine, just the aggravating kind that you try to ignore, but it keeps getting worse -- the kind that makes you irritable, so maybe that's why cigarette withdrawal has that irritability reputation.

Chain vape for a few minutes, it goes away.
I've had to walk a very fine line between too much, which makes me sick, and not enough, which has all the usual complaints of cigarette withdrawal, so I've gotten very familiar very fast with all those symptoms above. I find that vaping a lower level than you were used to with cigarettes actually seems to work best, because you can always vape more if you feel the need; but once you've taken too much, you just have to endure until your body gets rid of it.
Andria