I suspect she's probably had a laparoscopy (or two) for a medical concern and that's behind the altering of her belly button.
I don't follow the younger celebrities much but in the OP (her earliest shoots) it looks like Bella started off with a regular belly button with the usual nub in it. Weight loss would probably make that more prominent, but it appears as though that hasn't happened, rather hers has done a complete 360 in shape and gotten larger & more open. Either way, interesting button.
