The 18-year-old "Keeping Up With The Kardashians" star is not happy with her own family member betraying her, reported People magazine.
"Kylie is livid. She feels super betrayed and can't understand why someone in her own family would stab her in the back like that," a source said.
She has been dating the Tyga since late 2014 and Blac Chyna is his ex-fiance, and together they have three-year-old son, King Cairo.
On Monday, Chyna shared an Instagram photo of herself wrapped in the tattooed arms of Rob, announcing her new relationship with Jenner's sibling.