We broke the story ... Slocum -- who wrestled under the name Jackson Andrews -- was named in a police report filed by Mendes, in which she claims Slocum beat her up on multiple occasions in the home they shared together in Vegas.

But the story REALLY came as a shock to a woman named Amber Stovall ... who thought she was EXCLUSIVELY engaged to Slocum ... who she had been dating for the past 4-and-a-half years.

"I was not aware of his double life with Rosa," Amber tells us ... adding, "I'm incredibly angry and I feel betrayed."

Amber -- who says the two shared a home in Houston and have two bulldogs -- tells us, "I was blindsided by the TMZ story. He had just flown me out to Vegas the weekend after July 4th and we made plans for him to return home to Houston at the end of August. "

For the record, Rosa -- who's a WWE Diva -- was in Texas on the weekend after July 4th ... when he flew Amber to Vegas. Basically, he pulled off a fiancee switcheroo.

Despite the lies, Amber insists Slocum was never violent with her ... telling us, "He's never even raised his voice to me."

Obviously, Amber has since called off the engagement ... noting, "He has not reached out to me despite my efforts to contact him since I found out about Rosa. So as of today, I have cut off all joint accounts which includes his cell phone."