Amber Portwood and her fiance, Matthew Baier, come face-to-face with Gary Shirley on next week's Teen Mom OG reunion special.

'Teen Mom OG' Star Amber Portwood Feuds With Gary Shirley's Girlfriend On Twitter

While sitting down with Shirley to chat about the events of the recently-aired sixth season of the show, Baier takes aim at his fiancee's ex-fiance, and makes excuses for being a deadbeat dad to his numerous children.

“I think he’s full of s**t, to be honest with you,” Baier told host Dr. Drew Pinsky, via Radar Online. “His motivation for doing what he did was not what he said. If he had that motivation, he would have called her and said, ‘Hey, what’s going on with this?” Instead he purposely waited for the cameras.”

'Teen Mom OG' Amber Portwood Is Furious With Gary Shirley: Find Out Why!

During season six, Shirley confronted Portwood with paperwork, which reportedly confirmed Baier had at least seven children.

“It was more shame on myself because those were really painful memories for me so I kind of pushed them aside,” Baier explained. “Like, ‘He had children when he was younger, and he abandoned them and disappeared.’ That’s not what happened.”

Still, Shirley stands by his decision to reveal the documents to Portwood.

“When someone wants to come into my daughter’s life, whatever your motives are, the only motive I have is to be a father,” Shirley said of his decision.

Tune into the second half of the Teen Mom OG season six reunion special, airing on Monday at 10 p.m.