The 'On The Road' actress, who split from her boyfriend of more than four years Robert Pattinson last month, is reportedly very upset that their mutual friend Katy has been comforting the British actor and spending a lot of time with him alone recently.
A source told the MailOnline.com that the 24-year-old star feels "incredibly down right now and betrayed" at rumours of a romance between her ex-boyfriend and the 'Firework' singer, who have been close friends for several years.
The insider claims that Kristen has cut off all ties with Katy and still hopes Robert will want to get back together with her, explaining: "Kristen is in a certain state of denial. She can't let go of Rob or get over him.
"She keeps trying to contact Robert and find out what he is doing but her attempts to see Rob have been rebuffed. Rob is not responding to her."
Robert and Katy were most recently spotted together on Saturday night at Bjork's concert in Los Angeles.
It was recently reported that the singer, who has an on/off relationship with John Mayer, encouraged the actor to break up with Kristen after she cheated on him with married director Rupert Sanders last summer.
A source previously said: "Katy never really supported Rob's decision to get back with Kristen. She always said they wouldn't last.
"Katy told Rob she didn't think their relationship could withstand the cheating scandal. And he deserved better than someone who would destroy a family [like Rupert's]."