If you were not arrested for a new offense, this is a technical violation. Probation understands that sometimes people who use drugs relapse, even though they are required by law to lodge a violation against you. While probation clearly isn't thrilled about the positive, if this the first time you have been in trouble, your PO will probably not be looking to throw the book at you.
Probation has a whole arsenal of possible sanctions available to them, ranging from a wrist slap (more intensive supervision, more reporting requirements) on the one extreme to house arrest or revoking probation outright on the other.
What they will do in any particular case will depend on many factors, such as a defendant's background and history and the facts and circumstances of the underlying case, how well he's been getting along with his probation officer, whether he's been in good over all compliance with the terms of his probation before the violation or whether he's given them trouble before. You get the picture.
It will all come down to whether probation thinks that working with you will ultimately turn you into a successful probationer and a productive citizen or whether they think you never will get the message and will want to wash their hands of you.
In my experience, the fact that you voluntarily put yourself in rehab is in your favor and it shows remorse and a desire to get yourself help and under control. If you've had no previous problems with probation, they will likely just increase your reporting and your testing schedule.
You will be entitled to a hearing on the violation. You should have a lawyer at that hearing so that in the unlikely event that probation and the DA are asking for you to be resentenced to jail or prison, someone can make a good argument for continued drug treatment or house arrest.