Failed Drug Test

So I went to my psychiatrist today and we talked about be getting a prescription for concerta. I took one of my brothers Vyvanse on friday during one of my midterms and i didn’t know she would drug test me for it but it showed up on the test and she canceled the prescription. What do I do/say? I was honest before i took the test and told the doctor that handed it to me that i took one she said it probably wouldn’t show up but if it did it would be fine.