If you have never used cocaine, you certainly have the results of an improperly performed test. They're testing for the metabolic breakdown products (metabolites) of the drug in question. To do the job right they need to use a gas chromatograph. They vaporize a little bit of the sample and ionize it. Each substance will then emit light at a specific wavelength.
A few peaks on the chart that correspond to cocaine metabolites would explain the positive result. Could be though that they screwed up and got the wrong sample or that they didn't clean the instrument from a previous sample. Just have them check it again.
I'm not a chemist, but it's unlikely that a PH/AAS would give a false positive result, since none of the active (or inactive) ingredients should metabolize into the substances they are searching for.
For tested sporting events, they look for metabolites of known AAS, plus excessive levels of testosterone, and also the ratio of test to estrogen (and probably a number of other things I'm unaware of).