What Could Cause Positive Fentanyl In Urine Test?

I am in pain manangement I am perscribed 15mg oxycodone for a pinched nerve in cervical region and a slipped disk in another region of my cervical region.I have been with my Dr for over a year. I take my meds as perscribed, Last month when I went in tomy Dr they said I tested positive for Fentanyl in my urine test. Never and I mean never have I used Fentanyl. My Dr said well it showed up in your test I said go ahead right now and re-test me. Well they didn t and they cut my script. I understand thier situation but at the same time now I think I m a liar. Is there anything I can do. I see sometimes that peoples test get mixed up , etc. any help would be appreciated.