“TEEN MOM” AMBER PORTWOOD sits behind bars, as baby daddy GARY SHIRLEY gets hot and heavy with their 3-year-old daughter’s baby sitter!

And he’s even letting the tot refer to 21-year-old Kelsi Bowden as “Mommy”!

“Gary is a womanizing, drunken slob who only cares about himself while the mother of his child rots away in jail,” a source told The ENQUIRER. “He has no regard for Amber. She’s definitely going to end up suffering more heartbreak because he’s once again thinking of himself and no one else. It’s like he’s cruelly taunting Amber.”

In the new print edition of The ENQUIRER we have has exclusive photos of Kelsi hanging out with both Gary and his daughter Leah – and sources say they’re sure to drive jailbird Amber crazy.

Leah has grown extremely attached to Kelsi,” revealed the source. “When Amber finds out what’s been going on behind her back, she’s going to be devas­tated. Not only is Leah calling another woman ‘Mommy’ – that other woman is moving in on her man!

“Amber is already afraid that Leah is not going to remember her once she’s released from jail.”

An unlikely playboy, slovenly Gary has developed a well-deserved badboy rap for his out-of-control skirt-chasing and booze-fueled es­capades.

As The ENQUIRER reported in January, the 25-year-old had a drunken sex romp in an Indiana hotel room with a girl he’d picked up at a local bar – right in front of other par­tiers – before vomiting and passing out. His sex partner and her friend then took pictures of him before they left – after covering his naked body in Doritos and pepperoni slices!

At the time, Gary was still living with Amber, who thought he was at a friend’s house watching football!

In February, troubled Amber, 22, pleaded guilty to drug charges and probation violations and entered a re­hab program in lieu of jail. But by June, she had quit the program and was ordered to serve five years at Indiana’s Rockville Correctional Facility. Gary was awarded full custody of Leah.

According to the source, Gary has known single mom Kelsi – she has a son who’s Leah’s age – for about three years. He initially asked her to baby-sit so he could keep up his late-night partying.

“His behavior ramped up even more after Amber went to prison,” said the source. “And with Kelsi being around so much, they start­ed hooking up too.”

Gary and Kelsi have been spend­ing a lot of free time together, including overnight stays at each other’s homes. While rumors per­sist that Gary is still up to no good, the source says he does “care a lot” about Kelsi.

“And to her credit, Kelsi doesn’t want Leah to forget who her real ‘mommy’ is,” added the source. “That really impressed Gary. But the bottom line is he’ll eventually get tired of Kelsi too.

“Kelsi really needs to start thinking with her head and not her heart before she gets too at­tached to Leah. That little girl is the ultimate victim in all of this.”