Ariana Madix Mourning Her Dog When Tom Sandoval, Raquel Leviss First Had Sex

Ariana Madix is not keeping quiet about how ugly the cheating scandal with her ex-Tom Sandoval and Raquel Leviss went down -- revealing the cheaters first had sex in the car, while she was mourning the death of her dog.

In a preview clip of the season 10 finale of "Vanderpump Rules," Ariana opens up about how she found out about the whole 'Scandoval' for the very first time ... and how the call went when she first confronted Raquel.

If you already thought things were bad, the story gets worse.

Ariana says, she first found a screen recording of them on FaceTime, and, "That's when I called Raquel and I started, like, making her, like, f***ing tell me, 'What the f***? When did this start?'"

To her surprise, Raquel explained it had all started after a girls' trip to Vegas last August -- a trip Ariana left early because her dog Charlotte died.

To make matters worse, Ariana says Raquel still didn't tell the whole truth -- only confessing to kissing -- but later Tom Schwartz revealed Sandoval and Raquel banged in the car.

She adds, "And he didn't have a key to get in so he had to ring the doorbell and I had to let him in" ... yikes!

As you know, it's been a whole lot of drama involving the entire cast after TMZ broke the news Ariana had discovered the cheating was going on ... blowing up her 10-year relationship with Sandoval.