How could I have tested positive for Marijuana when I never used it?

I recently tested positive for Marijuana but I have never used any drugs. They retested the same sample twice but say that both were positive with a level of 143 ng. Again, I never used drugs so I know there must have been a mistake. I went and had another test a week later and it was negative. I am tested frequently because of my job and this has never happened before. What could be the problem? I am told that the positive cutoff is 15 ng. Mine was almost 10 times that. Wouldn't that mean that I would have to be a heavy or at least regular user? If that were true, how could I be negative a week later with the same testing method (urine)? I am going for a hair sample this week but I am dumbfounded by the whole thing. I was taking some OTC meds for a cold and ripped fuel 5x for weight loss but I was told none of those would produce a false positive.