Answer:
If it tested positive for opiates and you have not used any, there have been cases that indicate you can get false positives if you routinely eat a lot of poppy seeds and especially a whole lot before the testing. Opiate drugs are made from poppy plants, the seeds sold for use in cooking can register on the tests even though they do not really contain more than traces of the opiates, but the amount usually used in recipes or on breads isn't really enough to do that, you would have to be eating a LOT. If it were another type of drug that was positive, it depends on the drug that proved positive what might have caused that positive finding.
Contact buzz if you were around someone smoking marijuana and that is what was positive on the test. Apart from that, you wouldn't get a false positive on pot.
Aspirin will show up as cocaine. Cold medicine will show up as methamphetamines.