You could request your primary do a hair drug test- which shows results for months back up to current depending on the length of the hair sample.
Unfortunately, the time frame to prove a false positive is gone already.
Oh gosh. That's rough. I think the statistic is that drug tests produce false positives 5 to 10 % of the time. Which is a lot. Is this a pain management clinic? Cocaine is one of the drugs that doesn't have as many false positives though. Is it a one and done policy at the clinic? Will they not retest you?