Was she lying, why did she say this?

She's just trying to hurt you and it doesn't mean anything. Maybe your bf calls all pretty girls bella it's just a term of affection like i call young girls mija but that doesn't mean any of them are more special than others. She probably really wanted to know why you changed it to bella and since it was something nice she wanted to hurt you. Next time ask her if he called her faya (ugly in spanish) instead of bella?