I have NEVER taken anything other than what my Doctor at the time has prescribed.

I went to my pain clinic today and was informed that my last drug screen came back positive for suboxone. I have been going to this particular pain clinic for over two years and prior to that was being treated for long term pain control for an additional 3 to 4 years. I have NEVER taken anything other than what my Doctor at the time has prescribed. NEVER. I wasn't even for sure what suboxone was. About 5 months ago this same clinic said I tested positive for something else that I hadn't taken. What could possibly be the cause of this. Now I must find another pain clinic to treat me as without my medication I can hardly walk or move. Thank you for any information. I am beside myself as I know I haven't taken anything other than what was prescribed to me. Don't know if it's the lab they use or what; but why in heavens name would all of a sudden a 57 year old suddenly, after more than 5 years, start taking everything but the kitchen sink?!! I know I'm not guilty; now I just want some answers so I can show others I'm not too.