I do not have the chemisrty of the answer for you, but I may have a remedy.  You must contact the testing facility and give a complete list of all medications you are currently taking.  There may be an interaction that causes this undesired result. Also the body can store drugs for years in the fatty tissue of all humans. That would include mammary, and other reproductive organs. Perhaps "cleaning" your body has lead to the release of opiate metabolites into your blood stream and excretion through the kidneys. Just a thought...I would definitely contact the testing company before anything negative results. If as you say there is no reason for the "false positive", then do not wait to take action.