Why Does Drug Screen Test Show Positive Without Taking It?

Hello. I have been taking medications for pain for several years. Always going through a pain management dr because that is their specialty. In February I changed my pain management dr because I didn t think I was getting the level of care I needed. I found a new dr that not only prescribed medications but also did procedures which seemed to help me more than just being on medication. I have had several drug screens over the years all of which I ve never had a problem with. Recently one of my drug screens came back with methadone and morphine. I didn t even know what methadone was until it was explained to me. I have NEVER misused my medications or taken ANYTHING other than what has been prescribed to me. I guess my question is how would I test positive for medications that I have never taken?