LabCorp

The doctor called and said my urine was positive for marijuana in September but my obgyn told me i was not positive for marijuana in August. Im 13wks pregnant and take a urine screen once a month at my prenatal visits. And was never found to have any drugs in my system. They also use lab corp. So i dont under stand how I tested positive at this location. I lost out on a very excellent job offer because of this. What can I do to resolve this manner..? I'm very embarrassed because I've never used drugs . And also I was tested today 9/ 27/ 19.