Any other symptoms beside's nausea...like what hrdgain was saying? Lightheadedness, dizzyness, spots before you eyes?
Regardless, try sqatting down when it happens. I mean squatting and just being still, not getting under a barbell
. That might help better than just sitting down.
I don't know if it's blood sugar. I guess it could be. It could also vasovagal syncope...changes in cardiac output and blood pressure (you can google it).
It's more than likely a temporary thing and will go away on it's on but it this continues and you have those other symptoms it would be worth going to the Doc. Make sure the heart is good.