Amber Portwood Drug Test Violator

3/8/2012 2:00 AM PST

'Teen Mom' Amber Portwood -- Drug Test Violator

"Teen Mom" star Amber Portwood done screwed up again -- TMZ has learned, she failed to take a required drug test ... and could face the 5-year prison sentence she narrowly dodged in January.

TMZ broke the story ... Amber was sentenced to 5 years in prison after her December drug arrest -- but the judge promised to dismiss the entire case if she completes a drug program ... a stipulation that includes daily drug testing.

Amber's been doing great -- but on Monday, she torpedoed all her progress ... by failing to provide a urine sample during a routine drug test while she serves time on home detention.

Sources tell TMZ, Amber failed to provide a urine sample because she simply couldn't go at the time -- but life's not fair ... and no pee counts as a failed test ... no matter what.

Amber's due in court Thursday for a weekly check-up with the judge -- and she'll have to answer for the failed test. If he wants, the judge is within his power to throw the book at her ... locking her up for her original 5-year sentence.