But as far as I can tell, none of the other cpds should have caused that false positive. Someone more versed in chemistry may advise otherwise. If I had to guess, I'd look more towards a false positive from pseudoephedrine / ephedrine (adulteration?) in one of your supplements, and that's how I'd advise someone to contest the findings.
Again, I could be wrong, but right off that bat, that's my thinking. Depending on the repercussions, you might consider getting an independent lab to test your hair for methamphetamine and related compounds. Also. get a copy of the positive test, find out what the exact readings are, compare the readings and the protocol to the official corporate policy (ie. was the reading high enough to be a legit positive, did they test a second sample using a higher accuracy protocol...)