Life & Style says the "devastated" popster, 27, is questioning what they had together, now that the British funnyman, 36, has reportedly been seeing a brunette artist and model by the name of Oriela Medellin.
"I'm such a fool," the tab quotes Katy as telling a confidant. "I believed in him."
Brand, who renounced his bed-hopping ways when he hooked up with Perry in the fall of 2009, was snapped dropping off an evening-gown-clad Medellin, 25, at her Los Angeles apartment following an apparent overnight visit to his bachelor pad on Feb. 12.
According to People, he's been taking yoga classes with the fellow vegan and "modern-day gypsy" (her words), and they hit the town on Feb. 18, turning up at the Chateau Marmont.
Still, Brand appears to be keeping his options open and his flirting skills honed.
"He gets a lot of phone numbers -- the women just hand over their numbers and he takes them," a spy tattles to L&S.
Russell's rubbery rebound might explain Katy's blistering performance of the kiss-off song "Part of Me" at the Grammys.
"It's very disturbing to Katy that he's moved on so quickly," relays the Perry insider. "She was shocked to hear that he was out partying again and trying to pick up girls. It sickens her -- she feels humiliated and burned."
RELATED ON WONDERWALL:
- Jul. 14, 2016 See the stars' fashion hits and misses for July 2016
- Jul. 18, 2016 See all the of the biggest musical gigs of the summer!