Are you experiencing any kind of sinus/mucus issue as you ascend? I sometimes get that, and I imagine that could cause nausea especially if swallowed.
A little troubling that your instructor did not discuss more with you. If you are trying again next week, I'll just remind you that it is possible to vomit into a regulator at depth without too much problem -- hold the reg in your mouth if that happens, so it isn't spit out. If that reg gets clogged, remember you can switch to your own octopus if necessary (you can purge your main 2nd stage regulator while breathing from your octo, if needed). Just stay calm and don't shoot to the surface too fast in any event (even at shallow depth).