Troubled Teen Mom star Amber Portwood is back in an Indiana jail after local prosecutors claimed she lied about a medical condition to avoid a drug test.

As part of her probation, Portwood is required to undergo a weekly test and drug court appointment. She was apparently a no-show at last week.

Amber told officials that she was on doctor-ordered bed rest following laparoscopic gall bladder surgery and required to remains so for 2-3 weeks.

A good excuse ... if only it were legitimate.

Amazingly, Amber Portwood's case manager didn't want to take the reality star's word for it. After he spoke to the doctor directly, he learned she lied.

The doctor insisted she WAS well enough to attend her court obligations, and Amber was immediately hauled back to jail for missing the appointment.

Because of the obvious violation of her probation, she's expected to stay behind bars for at least a few weeks until a judge decides the next step.

This isn't her first drug test no-show ... or legal issue stemming from her own lies. She also underreported her income to score government-subsidized housing.

[Photo: Fame/Flynet]