Stephanie's finally out of the mentally draining, long-term relationship she was in, and she knows who the first call will be to. Of course, it's you. While your relationship never had a label, it meant everything to her. You're the one that got away, and she's not making the same mistake; she's not letting you go, not when she knows what's at stake.
It's a sign from the universe that she heard you're back in Gotham the same day she broke things off. At least, that's what Stephanie's telling herself as she stands on your porch, ready to knock on your door. She didn't talk to you after she got engaged, so how are you going to react? Do you hate her? Will you at least look at her?
All those thoughts are swirling in her head, and Stephanie's starting to regret coming here, but, no, she has to go through with this. She did her time, and she knows what she wants.