See Teen Mom Amber Portwood's Somber Mugshot

Credit: Splash News

Amber Portwood doesn't have much to smile about these days.

The 21-year-old Teen Mom star was arrested Monday for violating her probation. (In June, Amber pleaded guilty to two counts of domestic battery for assaulting her baby daddy Gary Shirley in August 2010.)

According to the MTV reality star's brother Shawn, "probation officers visited Amber for a random drug test at her house and they found pills in her purse that she did not have a prescription for."

Though Amber told officers the drugs had previously been prescribed to her, she also "missed a court hearing," Shawn told Us Weekly. "That is why she was jailed this time around."

The Teen Mom star -- mom to daughter Leah, 3 -- is expected to remain behind bars at Madison County Jail in Indiana over the holidays; she will stand before a judge January 13.

"Amber is distraught because she was planning to spend Christmas with her family," Shawn told Us. "Gary was going to let her see Leah also."