Taylor Swift knew Kacey Musgraves was trouble as soon as the brunette beauty hit the country music charts!

As Nashville’s hottest new leading lady, Kacey is knocking Taylor out of the top spot in the music biz – and also stealing her friends.

Taylor, 24, saw “red” when 25-year-old Kacey took home the Grammy for Best Country Album for her “Same Trailer Different Park” at this year’s show – an award Taylor was also nominated for – and she’s been a thorn in Taylor’s side ever since.

“Taylor is used to having all of the young country stars bowing down before her, but Kacey barely gives her the time of day,” said a source. “Taylor doesn’t like Kacey, and the feeling is certainly mutual.”

Taylor’s also stewing that Kacey has found a new BFF in “Dark Horse” pop star Katy Perry. The two women recently taped a TV special together, and Katy’s invited Kacey to be the guest act on her upcoming summer tour.

Meanwhile, the source said: “Katy and Taylor have been good friends for years, but Kacey seems like the ‘It Girl’ now. This is the first time Taylor’s been on the outside looking in at the ‘cool girls club.’ ” 

While Taylor has spent the last nine years carefully cultivating her girl-next- door image, Kacey stormed on the scene just last year, singing controversial tunes about smoking pot and same-sex romance.

“She’s hot, sexy and outspoken,” the source added.

“She’s making Taylor seem old hat by comparison. And Taylor certainly doesn’t like it.”