Luann de Lesseps is opening up for the first time about what led her to file for divorce from Tom D'Agostino, whom she was married to for only seven months.

In a sneak peak of a "Watch What Happens Live One-on-One with Luann de Lesseps," Andy Cohen sits down with the "Real Housewives of New York" star at her refuge in Sag Harbor to discuss what caused the demise of her seemingly-doomed marriage.

"It was like the weekend before the reunion, basically," Luann said in the clip above. "Tom went out and he called up an old girlfriend and they met up with a group of people, and I didn't know about it. I found out about it the next day in the press, so that for me was the final straw. I was like, 'I can't do this anymore.'"

Cohen then again brought up the Page Six article from July 16 that alleged Luann's marriage to Tom was "hanging by a thread" and that she slapped him in a restaurant in Palm Beach. Cohen originally asked Luann during a "WWHL" appearance July 19, prior to announcing her divorce, telling him at the time, "I did not slap him. We have a very passionate love affair going. And let me tell you, we never go to bed angry, but when we do, when we wake up, we make up."

Luann reaffirmed that she did not hit Tom, but the rest of the story became a bit more somber.

"I didn't slap him," she told Andy. "What I did was grab his face. It was in the heat of the moment, and I was hurt by something, so I just -- I wanted his attention so I grabbed his face, so it wasn't really a slap."

The reality star announced her split from Tom on Twitter on August 3.

"It's with great sadness that Tom & I agreed to divorce," she shared. "We care for each other very much, hope you respect our privacy during this sad time!"

Andy's full interview with Luann will air Wednesday, Sept. 6 at 11 p.m. ET/PT on Bravo.

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