When you're on probation, especially felony probation, you are not to commit a criminal against the laws of the state of texas. Smoking marijuana is a crime. You are being drug tested because the state wants to make sure you're abiding by your terms and conditions. You might not be revoked for it, but why would you even risk felony probation revocation- that means prison time for whatever time period you were sentenced to originally.
Be smart and make sure your PO knows you're serious about staying clean.
To add to Mr. Soechting's answer, the answer is that it is entirely up to the PO to inform the judge about any and all problems with your probation. The probation officer has complete latitude in this matter and can simply warn you or, as Mr. Soechting said, file a revocation. In between those extremes, the P.O. can file an admonishment where this is brought before the judge so that basically he/she can yell at you (and potentially send you to jail for a few days as "a condition of probation"), or file a modification, which is where the PO wants you to stay on probation but also wants to increase or change your supervision.
Be up front with your PO. They hear bs from people constantly and they appreciate it when you take responsibility for your actions and your probation.
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