Justin Bieber is not happy hearing or reading news about Selena Gomez dating Niall Horan. The "What Do You Mean" singer wants to know straight from his ex if the rumors are true. A source said Bieber has been desperately trying to talk to Gomez to get the truth out of her about her relationship with the One Direction singer.

A source told Hollywood Life that Bieber, 21, is "absolutely so upset." He reportedly can't believe that of all people, Gomez, 23, would be hooking up with his friend Horan, 22.

"He feels totally betrayed and is flipping out. He's been texting Selena non-stop, and she hasn't been responding," the source said.

The source added that Bieber is "desperate to get in touch" with his ex. After not responding to any of his texts, the "Sorry" singer reportedly took to his Instagram to try to talk to Gomez. Bieber has been posting throwback photos of him and Gomez back when they were still together.

The source's claims came after several eyewitnesses said Horan and Gomez were showing a lot of PDA at Jenna Dewan-Tatum's birthday bash on Friday, Dec. 4. The pair was among the guests at the star-studded party, which was held at Bootsy Bellows in Los Angeles.

An insider told E News that Gomez and Horan were seen hugging and dancing close together at the party. Meanwhile, another eyewitness told US Weekly that they acted like they were together. They reportedly stayed close to each other the entire time and were even seen "kissing in the middle of the club, in front of everyone."

"Niall got there first with his bodyguard, then the next thing I know, I looked over and Selena was there. They were very close together, talking and whispering," the source said.

Gomez and Horan reportedly left the party together. The "Same Old Love" singer was seen holding to Horan's arm when they left.

The One Direction singer also documented the event on his Twitter. Horan shared his photo with Gomez and some friends at the party. He has his hand on Gomez's shoulder while she makes a silly face at the camera.

Bieber, meanwhile, reacted to the news of his first love dating Horan by sharing another throwback photo. This is the second time he has shared his photo with Gomez back when they were still together.