Drug test showing positive for opiates?

I just did a drug test and I got a positive result for opiates. The test was repeated with the same outcome. The thing is, I have never taken any opiates. About six weeks ago, I was at a party where a lot of pot was being smoked but certainly not any opiates. I didn't smoke any pot. Is there any possible explanation as to why my urine shows traces of opiates?

: http://alcoholdrugstreatment.info/drug-test-showing-positive-for-opiates