Taylor Swift and Katy Perry won't 'shake it off'...the pop star princesses appear to be in a fierce feud. As John Basedow (@JohnBasedow) reports, the 'I Knew You Were Trouble' singer is rumored to have called out the 'Dark Horse' crooner for stealing three of her dancers, and Katy took to Twitter reportedly referring to Taylor as 'Mean Girls' character Regina George.