When Styles and his One Direction bandmates presented an award onstage, Swift, who was sitting with Selena Gomez, was filmed mouthing "shut the f--- up!" in the audience, reported Us magazine.
Swift, 23, previously mocked Styles by speaking in a British accent during her 'We Are Never Ever Getting Back Together' performance at the Grammy Awards in February 2013.
Later in the evening, the entertainer won big for her 'I Knew You Were Trouble' hit, which was reportedly inspired by her split with Jake Gyllenhaal.
"Thank you. Wow. I want to thank the fans because I tweeted about this a lot, I really really wanted this. I also want to thank the person that inspired this song, and he knows exactly who he is, because I got one of these!," she said in her acceptance speech.