I train cadets for 6 years now and i've seen many of nausea cases.
Most of them got over it pretty soon.
The ones that got rejected because of this, either had a health problem with their stomachs (or inner ears) or they couldn't stop thinking about it inflight.
So, if it happens to you continusly for 3 or 4 flights, first ask a doctor and have some tests.
If you're ok, the best advice i can give you is "Stop Thinking About It" and especially during flight.
You're up there to enjoy yourself, don't let these "i'll get dizzy" thoughts ruin it.
