Physician Toxicology review: Urine drug test

I'm Epileptic and in a lot of pain from many things. I had to go to a pain management office so I can get pain medication. With all of the seizure meds I take, I have to be very careful with taking anything else. So I gave a urine sample the first day in the office and they said come back in a month. I came back to the month appointment and was told that I had a dirty urine and was positive for cocaine! That's impossible! Never in my life have I ever tried any kind of street drugs, because of my Epilepsy Medicine. Now they won't see me again but I'm sure it's on my record! For other doctors to see! They wrote me another 30 days worth and said that was the last time. I believe that test is wrong. What am I supposed to do?