False positives or false negatives occur all too often so you're not alone. What action did they take at the Dr.'s office? Did they still refill your rx?
You can ask to be retested, as DS said. They could do a blood test. If they retest your urine, be sure you hand it to a staff person and watch them label it with your name and initial it.
It's a shame that people have to take all these precautions but mistakes happen all the time. I would insist on being retested because having a negative for Subs urine sample could mean you're diverting your medication. You want the record clear on that!
Did you ask to be retested?