Justin Bieber's latest romance rumors may come as a shock to most people, but they're especially surprising Scott Disick, since Bieber's latest hook up is reportedly the mother of his three children, Kourtney Kardashian.

Several sources have confirmed that the 21-year-old "What Do You Mean" singer and the oldest Kardashian sister have been hooking up. "Justin bragged to his friends recently that he has been hooking up with Kourtney Kardashian," an insider told Us Weekly

"Oh, yeah, that's definitely happening and has been for some time now," another source said. "Every time Kourtney and Justin are together they're extremely flirtatious. They text each other all the time."

Bieber and Kardashian, 36, even spend this past Friday night out together in L.A. at hot spot Nice Guy and then at the karaoke lounge Blind Dragon and were flirty all night.

"[They're] casually hooking up," a different source told Gossip Cop. While nothing between the two is serious, it does occur occasionally and they are having fun. They were first spotted back in October when they had dinner at The Nice Guy and then spent some time together at Universal Studios. A source then had even shockingly said that they had "hooked up," but that it was "just fun."

When Kardashian's ex Disick - who she dated for almost ten years but broke up with earlier this year when photos of him and another girl surfaced - heard these romance rumors, he couldn't believe it. Especially since Disick recently entered rehab for drug and alcohol abuse as a way to prove to Kardashian that he wanted to make these changes in his life for the sake of their three children.

"Scott is trying to get better, trying to turn a new leaf and now Kourtney is going out late night and hooking up with another bad boy in Justin," a source told Hollywood Life. "So Scott feels like it is a slap to the face. He feels that he is doing what she wants and she is now doing things just to make him feel bad."