EXCLUSIVE: Kylie Jenner Still Isn't Over Tyga, Not 'Serious' About Rapper PartyNextDoor

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First loves die hard.

Although Kylie Jenner is dating up-and-coming rapper PartyNextDoor, the 18-year-old reality star is still hurting over her breakup with Tyga, a source tells ET.

"She's still a little sore from Tyga -- he was her first real love," the source says.

WATCH: Kylie Jenner Goes on Bowling Date With PartyNextDoor, Gets Upset When He 'Deletes' Her

Kylie was definitely upset over Tyga hanging out with aspiring lingerie model Demi Rose at the Cannes Film Festival last month, according to the source.

"Kylie is livid about Tyga's behavior at Cannes," the source says. "This is the exact reason they broke up. He was constantly texting girls and lying to her about it. That girl, Demi Rose, was texting him during their entire relationship. So, when she saw pics of them together, she was livid."

As for Kylie's new relationship with 22-year-old PartyNextDoor -- who's signed to Drake's OVO Sound record label -- the source insists it's definitely not "anything serious."

"Kylie is just living her life right now and having fun," the source says.

News of Kylie and Tyga's split broke in May, and a week later, she was spotted with PartyNextDoor catching Russell Simmons' All Def Comedy Live in Los Angeles. But it already hasn't been smooth sailing for the two. In the Canadian rapper's recent Snapchat, Kylie accused him of hiding her on his social media, though he insisted he was "protecting" her.

... And PartyNextDoor quickly appeared to make it up to her by serenading her on the piano.

These days, Kylie appears to be focused on her business. On Tuesday, she Instagrammed sexy pics of herself modeling her and sister Kendall Jenner's new swimwear line for Topshop.

WATCH: Kylie Jenner Slams Reports Claiming Tyga Owes Her $2 Million

In addition, she just moved out of her $2.7 million home and into a $6 million mansion in Hidden Hills, California, last month.

"[Kylie is] loving having her friends over and decorating it," our source says.

Watch the video below for more on Kylie's lavish new home.

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