How can it change? Surgery? As we know she likes her sisters outtie.
Well, ironically, she's standing next to Karlie Kloss, whose outie had gone inbetweenie due to weight gain. Perhaps Bella is going inbetweenie due to weight loss. Or yes, maybe she did have surgery because she likes Gigi's outie so much. Although, I doubt there's a surgery to turn an innie into a outie, since it usually deals with hernia removal. It would be the first time Bella went under the knife, if you've seen older pictures of her face.
