The DA is charging you with a probation violations without the probation department initiating it? Odd.

Are they alleging you smoked after sentencing but prior to the test? It sounds stupid but folks do it. Also for future reference, if you know you are dealing with the justice system...you need to stop smoking until your involvement is completely over.

The only real defense here, if they are alleging you smoked after sentencing, is that you smoked prior to sentencing. Be careful with this though. Did the court, after your guilty plea but prior to sentencing, make any reference/give you any direction as to drug use?