Selena Gomez got emotional over Justin Bieber at Taylor Swift's birthday Credit: Michael Buckner/Getty Images; Steve Granitz/WireImage

There's always one! Selena Gomez got emotional about Justin Bieber at Taylor Swift's star-studded birthday bash at her Tribeca apartment on Friday, Dec. 13, a source reveals in the new issue of Us Weekly.

While fellow partygoers like Beyonce and Jay Z enjoyed beers, pizza, and sushi and mingled with pals like Justin Timberlake, Gomez, who has dated Bieber on and off for three years now, was reportedly getting emotional over the "Boyfriend" singer.

"She shouted, 'No one understands me! My boyfriend doesn't even understand me," an insider tells Us, adding that bystanders — including guest Sam Smith — "just stared [at her]." Find out more details about Selena’s emotional relationship with Justin Bieber.

Aside from the dramatic scene, which took place on the upper terrace, the rest of the party was having a "total blast," another source tells Us.

Other guests at the bash, which was complete with a karaoke machine and kicked off after Swift's performance at Z100's Jingle Ball, included Swift's BFF Karlie Kloss, Kelly Osbourne, 5 Seconds of Summer, HAIM, Nick Jonas and Olivia Culpo, Charli XCX, Chrissy Teigen, Shawn Mendes, Ansel Elgort, and Jaime King

The A-list crew posed for a snap with the birthday girl, who captioned the pic: "So this is being 25…. #WHAT #bestbirthdayEVER."