Moore's ex, rapper Romeo Miller, wasn't happy with the "Boyfriend" singer's flirty message. He fired back by posting a snapshot of him and ex Moore, calling out Bieber with the message: "@justinbieber sometimes I wonder too..."

And Gomez joined in and backed up Miller by liking his post.

(Photo : Instagram/Romeo Miller)

Yikes, is this the start of yet another feud between the "Come And Get It" hit maker and her ex-boyfriend?

Bieber has been linked to several women since he and Gomez called it quits last year, including Kendall Jenner, Yovanna Ventura and Hailey Baldwin, among others.

However, this is the first time that Gomez has publicly reacted to Bieber flirting with other women. And a source has revealed toHollywood Life that there's a reason behind Gomez's apparent hostility.

"Selena still feels like Justin cheated on her with Ashley while they were still dating. So to see Justin publicly flirting with Ashley makes her skin crawl," claimed the insider.

The source continued: "She got a big laugh out of Romeo putting Justin in his place. She thought it was f-ing hilarious!"

"She's like a detective. She tries not to look, but she just can't help herself. It's not healthy and she knows that, but it's something she still does," another insider previously told the tabloid.

Despite the feud brewing between them, friends of the "I Want You To Know" singer are concerned that she might be considering reuniting with Bieber.

Another source told Hollywood Life: "Selena's friends wonder why she's still so caught up in what Justin is doing. They worry that she's falling back into her old habit of obsessing over what Justin is doing and saying. It's like she's still invested in the relationship even though it ended months ago." 

This article appeared originally on fashionnstyle.com