Why did my test come back positive?

I'm on probation, and I've been clean and clear from drugs for two years. Well this morning, my probation called me and told me that I tested positive for marijuana. I'm trying to figure out, how in the world did this happen? I don't smoke weed at all, and I'm just all the round confused right now. What could have went wrong? Are they're tests wrong? I know I haven't done any drug. So, he told me that, if I know I haven't smoked anything then I have nothing to worry about. They're going to send my urine to the main lab and test it.

Some organizations use screening tests that are quicker, less expensive but have a higher number of false positives. Wait for the final results.