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
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'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...