Benzodiazepines rarely cause nausea, but anxiety can. It sure did with me.
Xanax was not designed to be taken regularly for any length of time, and if you do then you will develop a tolerance to it. That means you either have to increase your dose to maintain efficacy. In other words, if you are building tolerance right now you're likely seeing a return of anxiety symptoms, which could explain the nausea.
If the Xanax has helped you then you should think about
discussing a long-acting benzo such as Klonopin or Valium with your doctor.
My Brain: My friend, My enemy: A blog to chronicle my attempt to recover from anxiety/panic disorderanxietypanicdisorder.blogspot.com/