The two main causes for nausea after rolls are
problems with your vestibular system (balance organ in your ear)
problems with your blood pressure adaptations (orthostatic nausea)
The first could be ...
It's important to identify the cause of the nausea.
Is it fear?
Is it incorrect breathing technique?
Is it fatigue (too many ukemi for a beginner)?
At my Aikido Dojo an advanced student was placed in ...