What Causes Positive Results For THC?

Hello. I recently saw my obgyn about two-three weeks ago and he did some standard lab work including a urine test. Well I checked online for my lab results a few days ago as they had just been posted and it stated that I tested positive for THC. I don t understand how as I don t smoke and all my previous lab work didn t test positive for anything . The only things I am taking are prenatal vitamins, Zyrtec, and prior to that visit I had taken some aleve here and there for severe back pain and I no longer had Tylenol. Well today I paid to have my own UA sent to a lab and I should have the results back Monday or Tuesday. How do I approach my obgyn on the subject? Does he have to acknowledge the lab work I paid for? Can I have his test results dismissed? Can I stop anyone else from seeing that one lab so that I don t get in any trouble when having my baby? What do I need to know? I m very nervous on the subject.