I Recently Tested Positive For Several Drugs When I Took

I recently tested positive for several drugs when I took a urine test at my pain management clinic. I can't rememer them all, but two of them were oxycodone and methadone. The only medicines I had taken were the prescriptions that they and my dentist gave to me, hydrocod/acetam 5-325mg and amoxicillin 500mg. He treated me like a druggie, which I am not. What caused this false positive reading? Exactly what other readings could these two prescriptions have given? What can I do to prove that I did not take the other drugs that his test says that I did?