The Xtra Factor host is alleged to have fumed about being kept apart from the One Direction star at Sony's Brit Awards afterparty in Mayfair on Tuesday (February 21).
According to the Daily Star, Flack was heard complaining: "Why aren't I allowed to speak to him? Why can't I speak to him?"
It has also been claimed that organisers demanded that a picture of Flack and Styles together at the event be deleted from a photographer's camera.
"Caroline wasn't on the guest list. A friend must have got her in," a source claimed. "The One Direction camp weren't thrilled when they saw her. They were not keen for an ex-lovers' tiff to distract from the celebration."
The pair confirmed the end of their short-lived relationship last month, with Styles writing on Twitter: "This was a mutual decision. She is one of the kindest, sweetest people I know. Please respect that."
The romance attracted criticism due to the age gap between 32-year-old Flack and Styles, who was 17 while they were dating.
> One Direction's Harry Styles: 'I'm through with older women'
Watch One Direction in the video for 'One Thing' below: