This nausea happened to me one time after my denture was relined and came back from the lab. The problem was this -the backof the denture on the roof of the mouth had to be slightly smoothed . I couldn't believe the difference. If there is too much material there it can make you nauseated as it is hitting some nerve. Have the dentist trim or smooth that area. This is what my dentist did to get rid of that feeling.