It could be- for a couple of reasons. Is this the same Dr who prescribed the Xanax and if not is he/she aware you are taking it and the dosage?
Most PM doctors will not prescribe Xanax or Valium type drugs at the same time as a patient is taking opiates. IF this Dr did not prescribe the Xanax to you and it is not a medication he/she is aware you are using - it would be a definite violation of a contract. If the PM Dr previously prescribed it but discontinued it as part of your current medications and you are taking it anyway- that would also be a clear violation. If you obtained it deform another doctor and your PM Dr is unaware- most PM Drs would also consider that a violation.
As far as the legal issues are concerned your best option is to tell the DR upfront and be honest and transparent. Some Drs will dismiss a patient who is arrested for DUI - so I would bring the arrest report, and the toxicology report and be honest and hope that counts in your favor.