I have a question about a false drug test that I received! Plz help,I will reward points as soon as possible.?

I was told to go to a local hospital and give a drug screen for a job. I do not do any drugs and also in college. I stay away from drugs and very seldom do I drink alcohol! The drug test was taken and tested in less than an hour. They told me that they found meth(methaphetamine)SP? Anyways, I told them that I have only seen that stuff once when I was younger. I told them that maybe the test was wrong or that they mixed up my urine with someone elses. They told me that I was the only male taken a test at the time and said there was no chance that it was mixed up. I argued with them for a while explaining that I do not do ANY drugs. They told me they would test it again and when they did it showed up the same. Can someone tell me why in the dong-kong did it show that I had that in my system? Honest to God, I do no drugs and stay away from people that do! Also,I only take zoloft for a medication.