I've heard of poppy seeds giving a false positive for opiate use, but am at a loss as to what might cause what you describe...
I think in some situations you have the right for a "retest" - now they may demand you pay for it, but depending on why you were tested initially, it might be worth it just to clear things up. Good luck to you!
I take pain meds and when I took a urine test at the Dr's, nothing showed up. Idk why but nothing. I think everyone's body produces things in diff ways