Ariana Grande was allegedly a "nightmare" during the Stand Up To Cancer fundraiser on Friday night.
The 21-year-old singer took to the stage at the annual event and an insider claims she was difficult to work with because of her diva antics backstage.
A source told the Daily Star newspaper: "Ariana was a total nightmare. At first, it was her warning the lighting crew if they used red spotlights she would go berserk. But then she kept everyone waiting while other stars came in and did their thing."
The source continued: "We've been on set with Lady Gaga, Madonna and Mariah Carey, but combined they aren't as rude as she was. When she caught one poor guy grimacing, she barked, 'Who the f*** do you think you're looking at?'
The insider added: "The whole episode was kind of surreal because once Ariana had calmed down, she was all sweetness and light. But it wasn't long before she stormed off again, unhappy with her outfit."
The brunette beauty, who lost her grandfather to cancer recently, gave an emotional performance of the title track from her new album My Everything. Taking to Twitter after the show, she tweeted: "Sobbed my eyes out as soon as I got back. I don't know how he was just here and now he's not and I'm singing a song in his honor. Sucks a lot #f**kcancer."