Sunday, April 7, should have been a great night for Taylor Swift. She was nominated in five categories at the Academy of Country Music Awards, she had a gorgeous scene-stealing gold dress for the red carpet, and she was scheduled to perform with Keith Urban and Tim McGraw -- a star she once sang about in her first-ever single, "Tim McGraw." But things took a turn when the "22" singer discovered that John Mayer was slated to take the stage first at the Las Vegas event, sources reveal in the April 29 issue of Us Weekly, on stands now.

One insider says Swift, 23, put on "a stink" about her ex-boyfriend's performance. "She said she had to go on earlier than him," the source tells Us of the songwriter, who had a brief romance with Mayer in 2010 and later wrote the song "Dear John" about him.

The country star got her way in the end, but a second insider says Swift "was just really bitter that night" -- especially after she didn't win anything. Adds another source: "John just being there irked her."

For more on Swift's "bitter" night -- including exclusive details on what she did after the show -- watch this week's Hot Stuff video, as explained by Us Weekly entertainment director Ian Drew and special guests Joseline Hernandez and Stevie J, of VH1's reality series Love & Hip Hop: Atlanta.

Other stories in the roundup: Robert Pattinson gives Kristen Stewart a 23rd birthday to remember; Ozzy Osbourne reveals a shocking secret; and much more!