The girl came home from work and received a text message.

From the linguistic point of view, "good night" and "An 'an" are not very different in expression, and they are both words of mutual blessing. But emotionally speaking, "An An" usually has a kind and warm feeling, and is sometimes used to express intimate relationship. Therefore, girls suddenly say "An An" to you, because they want to express more feelings, or because they think the relationship between you is closer. But the specific reasons need to be judged according to the communication and relationship between you. I suggest that if you are interested in her, you can respond to her appropriately to further deepen the communication between you. But if you don't want to pursue her, you can respond to her politely, but don't mislead her.