Most probably things didn't work out well with Option 1. So, now you are someone who was interested in her, so she is trying to be closer to you. To be honest, if you like this girl (before her saying "no"), then maybe this is not the worst thing in the world. I mean she might fall in love with you over time and everything, even if you were not her first choice--if you can handle the ego let down, I think this is not a terrible situation.
But having said that, I think you should definitely ask the girl and be like "so, last week you didn't want to talk to me, and now you are being all friendly. I am confused" and see what she says. She will probably make it clear that I just realized that I like you or she will have to explain herself in any case. And then see, if you still want to be with her.
Give it time, and then make up your mind. And the end of the day, try to look at the positive side, maybe you guys were meant to be, and so the other guy didn't work out.
Hope this helps-:)
Take care and good luck!