Long Time Pain Mgmt Patient

I've been a Pain Mgmt Patient for about 12 years.  I recently had a urine test come back positive for Fentanyl, something I had never heard of nor taken.  I've been taking MS Contin Extended Release 100mg, 2 every 8 hours with MSIR 15mg, 2 every 4 hours as needed for breakthru pain for about 6 + years.    I also take meds for high blood pressure, depression, water pills, anti-inflamatory, indigestion . . . . I think thats it.  Does anyone know how this could have happened?  Or could anyone explain to me what a false/positive means on a blood or urine test and how that is determined?  Your input would be greatly appreciated.  I'm really baffled here.