The 23-year-old and Perry were partying in a club when the 'Fireworks' hitmaker told Stewart that she would want to date Pattinson, reported Sun Online.
"Kristen looked like she wanted to run away after Katy's comments," said a source.
Stewart is said to have then questioned Pattinson whether "rumours of them (Perry) hooking up in the past are true".
Stewart and Pattinson had split after her cheating scandal with married director Rupert Sanders became public. But it is said that the couple have reconciled their romance post the much publicised scandal.