Note that this is different from a regnal name, which would generally be a first name. She uses Amidala when she is queen, but also when she is a senator, so it seems to behave as an assumed surname (in 'Senator So-and-So', the 'So-and-so' part is usually the senator's last name). Going just by the movies, it's not entirely clear what role each name plays, but given that her other family members use Naberrie, that seems to be her birth family name (surname). Padmé has three names: Amidala, the aforesaid Padmé, and Naberrie. Even on Naboo, it would be very strange to elect someone named, say, Mary, only to end up with an elected official named Elizabeth. It seems likely that when she served as Princess of Theed (basically, governor) and then ran for and was elected Queen, she used the name Amidala.
