i don't know the specific rules of your area but anywhere I'm familiar with random drug testing is always possible the entire period of your probation. On top of that you screwed up by forgetting to call. I suspect your po sees that as you trying to dodge a random drop likely because it would have come back dirty.
Feel blessed all that has happened is you are in an outpatient rehab program. S/he could always violate you for failing to comply with the rules.

