Same thing happen to me.
I have a medical condition for which I took pain relief without incident off & on for a number of years, as needed, at home, after work, & stayed at home once I took my med (just because I know that is the right thing to do, not that the med made me loopy or unsafe). The following day I was in terrible pain from a developing (new) medical incident. I was reported as impaired, had to do a UA which revealed the previous night's med. A ER doctor evaluated my new condition & could not believe that I remained at work for an 8 hour shift considering the shape I was in, but my UA was positive and "that was all she wrote." No one would even consider that the reason for my guarded movement was due to the new DX.
Was told to report to BRN within 48 hours or they would do it for me (which would make things worse for me). Discussion with the BON was hostile, midevil, and traumatic. I felt as if I was gutted with a dull knife. They gave me the option of diversion or meet with the state district attorney for more severe sanctions against me & my license.
Spoke to an attorney who told me that since I was not an addict (never had the obsession for the drug or the compulsion to use), that the not using would be the easy part, meaning that I would not have concerns of testing positive regarding the state mandated UA',. but everything else would be absolute hell, which it has been. Loss of the dream job, lost everything, BON losing my paperwork which I send in on time; I swear that they must attend a private seminar to find new ways and anything under the sun to make your life a nightmare. I wish I had more than 48 hours to explore EVERY OPTION available to me before I self-referred to something such as this torture. Not only is the loss of income and job, but what it does to your sense of esteem and confidence.
RUN, don't walk to an attorney. The link above is good=TAANA. Don't let anyone harm you.