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The POA doesn't really cover this transaction - your mother called you, and you then authorized a transmission of healthcare information between your clinic and the pharmacy.
You did not share your mother's Personally Identifiable Health Information with an entity not involved with the provision of her care.
(It is likely that if your mother was another patient, and called in the same thing would have taken place - assuming you have the authority to resubmit prescriptions to the pharmacy from a patient's chart).
What your mother's POA does cover:
A "Power of Attorney" should be on file with your mother's healthcare providers (including the clinic you work at), but the POA does allow the clinic to share information with you. See: http://www.hhs.gov/hipaa/for-professionals/faq/220/can-i-access-medical-record-with-power-of-attorney/index.html