'Celeb Rehab' Star -- Back In The Slammer

Law enforcement tells us ... cops stopped Frankie for a routine traffic violation yesterday when they noticed an active warrant out for her arrest stemming from a probation violation.

We're told cops then performed a search -- and found "drug-related material" on her person.

Lons was arrested and transported to a nearby jail -- where she was booked for the probation violation and misdemeanor possession of drug-related material.