I'm prescribed oxycodone but my doctor says my urine test shows hydrocodone, how is that possible?

I don't know how it's possible, but know it IS possible because it happened to me. My test results show hydrocodone (low amount), oxymorphone and oxycodone. I'm only prescribed Oxycontin and only take Oxycontin. I was told I "passed" the test. I believe your doctor is not understanding how to read the test.

On my test results paper it says "The presence of more than one opiate in urine may reflect drug metabolism or use of multiple drugs. Low concentrations of an unexpected opiate in the presence of large concentrations of another opiate may also reflect impurities in the pharmaceutical preparation" and

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"Hydrocodone may arise from hydrocodone-containing drugs or by metabolism." And - "Oxycondone is not a recognized metabolite of other opiates and its presence indicates use of an oxycodone-containing drug. Oxycodone is metabolized to oxymorphone." That explains the oxymorphone in my test result, but I don't understand the hydrocodone showing up, other than perhaps like it says above, "by metabolism" or "impurities" in the preparation.

I was trying to find out why hydrocodone was in my result too, but since it didn't concern my doctor, and she's very very strict, it must be something else, but exactly what, I don't know. I DO know I haven't taken any, and only take what's prescribed by her. I did read that many doctors do not know how to read the results, and those little additions that I posted here, was to help the doctor understand the results better? I'm still trying to understand it myself, even though it does explain the oxymorphone, it's not real clear about the hydrocodone other than "by metabolism"?