I Went To The ER Last Week And One Of

I went to the ER last week and one of the tests they did was a urine drug test. Today I get a certified letter dismissing me from my primary care physician. When I called to ask why, I was told that the recent drug screen came up negative for the Lorazepam I am prescribed that they refill every month. I take this medicine on a daily basis. Is there a chance that could have been a false negative?