Selena and The Weeknd created a stir when they shared a passionate embrace whilst on a dinner date at popular celebrity haunt Giorgio Baldi restaurant in Santa Monica, California, last week.
And sources close to Bella - who unfollowed Selena on Instagram after the pictures surfaced - later told TMZ: "Giving Bella and Gigi a heads up would have been the respectful and honorable thing to do ... but I guess that's just Hollywood."
They went on to explain that Bella feels betrayed because her ex and Selena hooked up almost immediately after Bella and The Weeknd ended their 18-month long relationship.
Meanwhile, it's been reported that Selena doesn't feel guilty about her and The Weeknd's blossoming romance.
Acccording to US reports, Selena insists she has no reason to feel bad because she and Bella are merely acquaintances.
Insiders told TMZ that although they are both part of Taylor Swift's squad, Selena and Bella have never been close and she didn't feel the need to inform her about the new romance.
While another source told E! News that Bella is still in love with her ex.
"She is actually not over The Weeknd. She still loves him. They are on fine terms, but she is bitter about the romance with Selena. She was not happy when all of that gossip went everywhere between The Weeknd and Selena.
"It really hurt her seeing Selena be all up on her man. She still feels like they have a connection."