I got a question that keeps me up at night

Is she out as gay or bi? If not, then it's extremely likely she's just not ready for a relationship with another girl or isn't ready to accept her feelings.

If she is out, that doesn't mean anything about what she thinks of you.
When someone rejects you, it's easy to think they did so because you're not enough for them or they don't like you enough. Obviously, that's not the case because you guys are such great friends and still are. So that leaves some other possibilities:
Maybe she doesn't want to wreck your friendship.
Maybe she wasn't clear on what the one night stand meant and how serious it was.
Maybe, while she likes you a lot as a friend, didn't feel like a romantic relationship felt right with you for whatever reason.

All these possibilities are very likely. And even though it was your first time, I'm sure if she was looking for a more romantic relationship with you that wouldn't stop her because she'd be understanding. Even then, I'm sure it went fine anyway.

I'm sorry you have to go through this too. It wasn't fair of her to enter into a relationship with someone else without explanation. But her friendship is proof that she likes you a lot, if not in a romantic way, then as a friend and she cares about you. It's also easy to think the worst-case-scenario is true when you don't have answers so try to remember that and that you really don't know why she did that so you can't assume the absolute worst. She probably wouldn't want you to feel that way if she knew.