What Causes Positive Drug Test Results For Codeine While On Subsolv?

Hi my name jacob. I take the medication subsolv prescribed by my doctor for opiate addiction and I get piss tested every month. I have been clean for years. Today I went in to get a refill on my subsolv and my piss test results came up positive for codeine. I have not taken any medication besides over the counter meds like aleive and Tylenol. I was really upset. What could of put this in my system?