What caused me to test positive for cocaine?

I have to take a hair test every 90 days. I have had three so far, and I tested positive for cocaine on the last one. I demanded another test and it came back positive as well. The only things I did different before that test from the ones I passed was that I had some dental work done using lidocaine, and I had been sick and I used some OTC meds. Please help. I swear I didn’t do cocaine, and I could have to go to rehab and lose my job over this.
My dentist told me he used lidocaine, but said he doesn’t know much about drug testing. The people in the drug testing industry refuse to ackknowledge that anything could be wrong with their tests, so I guess I’m going to rehab for something I didn’t do…