I am not sure why you would lie to your probation officer and tell you that you were drinking, when you hadn't been drinking.

You need to explain the nature of your OTC medication to your probation officer, and document that it can cause you to appear to have a positive BAC. You would also need to explain that the officers presumably did not observe you for a period of time before administering the PBT, and presumably that they didn't ask you about any medications that you were taking.