"So during that time, yes, I was in relationships with someone else," the reality star said. "That was when I started like, you know, drinking... medicine. I would push people away, I'd move them out, I'd get so angry. I'd just, like, break stuff. And I was in another relationship."
The mother-of-three fitness trainer claimed she then discovered Josh and her cousin had been allegedly texting back and forth repeatedly around this time. She ended up accusing Josh of cheating on her in a since-deleted Facebook post.
"So I saw the phone records and of course the first thing I did was go to the internet."
Although Mackenzie denied she ever said the pair were "banging," as claimed in one report, she explained, "I truly, in my heart, believed it was an emotional affair."
"It really caused a lot of drama," the "Body by Mac" founder continued. "Like this year, my family canceled Christmas because of me -- because of me, because of what I did. And I was like, 'I never said you two were banging. I said you two were texting and it wasn't ok.'"
"And so it was a scandal that blew way out of proportion," she added. "And I still think that he shouldn't have went to a woman for advice about me. And I don't talk to her and she's dead to me. Josh was dead to me at first too."
According to Mackenzie, she, Josh and her cousin were all friends at one point and the family was "close-knit."
"But after that, I don't speak to that side of the family," she said. "I don't speak to her. So, you know, I hate her. She hates me. It is what it is. And at that point, I planned to hate Josh for the rest of my life."
The plan didn't stick, however, as Mackenzie and Josh reconciled, with Josh moving down to Florida to be closer to her and their three kids: son Gannon, 9, daughter Jaxie, six, and son Broncs, four.
"I think he's in a spot where he's very, 'This is my family. This is where I belong. I made my mistakes and it made me realize where I belong,'" she said of Josh. "So now we're just kind of letting things play out."
The new season of "Teen Mom OG" debuts Tuesday, January 26 at 8PM ET/PT on MTV.