Its federal probation for possession of marijuana on a military base as a civilian

They can revoke your probation, lock you in jail till a hearing and put you in prison for all of the original sentence. That is the worst that can happen.

Since you were arrested for drugs and a dirty urine shows still using drugs, most likely they will revoke probation put you in jail. You will have a court hearing, after being in jail for a few weeks. And they will order a new drug program and closer testing and put you back on probation. ** my guess.