Taylor Kinney's ex-girlfriend is calling him a cheater.

In an exclusive interview with the Daily Mail, Brittany Sackett claimed that Kinney cheated on her with his now fiancée Lady Gaga while they were still dating. She said learned about the affair when Kinney accidentally called her while he was making out with the "Bad Romance" singer.

"Taylor pocket dialed me at 2:30 in the morning," the 30-year-old recalled. "I could tell it was intimate conversation, I could hear kissing, muffled sounds and I recognized Lady Gaga's voice."

Sackett said she "knew something was going on," but didn't know how far in the relationship Kinney was at the time, adding, "You have your instincts as a woman. Whatever he was doing shouldn't have happened while we were in a relationship."

Kinney's ex-girlfriend told the Daily Mail that at the moment she felt betrayed and distraught. Kinney and Sackett were dating for over a year, when he and Gaga met on the set of her music video for "You and I." According to Sackett, Kinney told her that he and the singer were "just friends."

Sackett said she kept quiet about her discovery, but eventually Kinney and Sackett ended their relationship. 

"In that moment I was super sad, I was pissed. I never saw it coming," she explained. "My boyfriend was making out with Lady Gaga - it sucked. I felt betrayed, who wouldn't? I wanted to see how it played out, but later I saw one of his friends and he said to me, 'You know what's going on with Taylor and Lady Gaga right?'"

Sadly I did," she added. "Eventually I met with Taylor and he said he was really sorry but I didn't want to hear any of his excuses. I told him how I felt, I told him he should have been honest so I didn't have to find out through his friend. We talked about how we weren't a match for each other and we agreed to go our separate ways."

Sackett may still be heartbroken because she has been sharing old photos of her and Kinney on her social media page. The real estate agent claimed that Kinney confessed to the infidelity via text message.

"He wrote that he was really 'truly sorry,' he said 'you're an amazing woman I'm so sorry I hurt you,'" she said. "It was really unexpected, everything was going well and I wish he had come to me first and said he had this connection with her. It wasn't fair to me, his lack of honesty. It's weak-willed when you don't have the guts to come clean like a normal, honest human being, I think I would have respected that."

Kinney and Gaga announced their engagement last month following a romantic Valentine's Day proposal.