Fans of Ex On The Beach saw Mel surprise Gaz as another of his exes on the show, and she was quick to reveal that she had slept with the Geordie Shore star while he had been dating Lillie.
Gaz then had to break the news back at the villa to Lillie and she was left feeling "embarrassed" again, realising that she had actually enjoyed a night out with Mel (after Gary had bedded her) and had no idea.
Speaking in a new video shared by MTV, Lillie criticised Melissa for having no respect towards her.
She said: "Mel has arrived - another girl Gary has slept with that I had no idea about... but this time it is actually worse. I actually saw Mel on a night out with Gary, after they had both slept together behind my back, and she was nice as pie to me. She was lovely to me and actually spent most of the night with me. Yet, this girl had slept with my boyfriend behind my back. That's a whole new level of disrespect."
Mel, meanwhile, wasn't sorry and when she came face-to-face with Lillie, she claimed she didn't "owe" her anything when asked whether she should apologise.
Still insisting she isn't the on in the wrong, Mel added in the clip: "I knew that Gaz cheated on Lillie with me, but I didn't think they were that serious, and I didn't think I was doing anything wrong."
She also revealed how her tryst with Gaz came about, revealing: "Every time I was DJ'ing at a club and Gaz would be doing a personal appearance, we would end up sneaking off to a hotel room together. I knew Lillie was in the picture, but I didn't know they were actually together."
Gaz claimed his hook up with Mel was "right at the start" of his relationship with Lillie. He and Lillie had dated for nearly two years, before he split with her in November 2015.
Not surprisingly, Lillie had broken down in tears after seeing Mel got a little too much for her. But, in a strange turn of events, it was actually her ex Gaz that was comforting her.
He vowed to stay by Lillie's side and tried to calm her down, telling Lillie that she is the "best person he has ever met." Lillie and Gary then ended up in bed together and viewers saw the exes kiss.
Explaining *that* late night smooch, Lillie had said the morning after that she "hit rock bottom" and "needed comfort."
She said: "Last night, I think I hit rock bottom and I needed someone to comfort me who knew me and the only person in here I know is Gary.
"I think whatever Gary does to me I'll always love him in some way. You can love someone but not want them and I think maybe that's who Gary will always be to me."
Gary's confession that he had cheated on Lillie with Mel came after he had previously admitted to sleeping with Chrysten Zenoni behind Lillie's back. Initially he denied anything had happened between himself and Chrysten, but was later caught out when Chrysten revealed she and Gary had had sex.
At the time, Lillie branded Gary a "horrible person" for betraying her.
Ex On The Beach continues Tuesday 17 October at 10pm on MTV.
Follow us on Twitter @RevealMag for all your celeb, fashion and beauty news!
Fancy winning some GREAT prizes? Check out Reveal Competitions here!