Nausea can be related to conditions of the brain or any of the organs of the gastrointestinal tract.Nausea and vomiting are not considered diseases on their own but are symptoms of another condition.
Some of the causes would be:(1) side effect of medication (2) motion sickness- usually may have additional symptoms such as vertigo or dizziness (3) gallbladder
disease (4) brain tumor
(5) gastroparesis-delayed gastric emptying due to partial paralysis
of the stomach resulting in food remaining in the stomach and vomiting (6) colonic or stomach cancer
I hope this helps and that she feels better soon