What if i smoked before i got unsupervised probation?

Ok so a few weeks ago i was charged with a misdemeanor and before i was charged i had been smoking. I was told i might get put on unsupervised probation, but what will happen if i get tested and fail even though its been a few weeks since i smoked and it was before the charges even happened? My court date is in a week or two and they said i might get unsupervised probation for a month and 20 hrs of community service. Please no smart remarks i'm just looking for some advice.. thanks!