"She feels like it is a complete stab in the back. Girlfriends don't do this to one another," the insider said. The Hills alum is also said to have blocked the country singer on social media.
"It took her all of one second after learning the news that she was seeing Jay to block her on social media," the source explained.
This comes after an insider told PEOPLE that Cutler and the 37-year-old went on one date, noting that Kramer and the Common James designer are only acquaintances. However, another insider told the publication that the mother-of-two leaned on the 34-year-old during her divorce from Mike Caussin.
"Jay was trying to make Kristin jealous by going on this public date with Jana. He's not happy she's dating Chase [Rice] and has been trying to get Kristin back," the PEOPLE insider explained.
"No one knows this but Kristin actually recommended her divorce attorney to Jana when she needed help leaving her toxic marriage," the source told the Daily Mail. "Both girls were really close, but Kristin cannot see a way forward in their friendship after Jana did this to her."
The "Whine Down" podcast host and the former footballer called it quits for good after six years together. She filed for divorce in April and cited "inappropriate marital conduct, irreconcilable differences and adultery" as her reasons for separating.
The former flames previously called it off in 2016 when Caussin was reportedly unfaithful. They got back together when he sought therapy for sex addiction. They share children Jolie, 5, and Jace, 2.
Cavallari and Cutler share children Camden, 9, Jaxon, 7, and Saylor, 5.
The reality star was spotted on a date with Rice on Monday, September 6, at Losers Bar and Grill in Nashville. The source told PEOPLE that they "were very affectionate and making out" but "it's still early days and they're not exclusively dating, but they're having a good time and enjoying getting to know each other."