I'm experiencing something similiar with TS. Confirm the girl has step out from the comfort zone already, she no longer feel comfortable with the relationship with you. She might have some interest on you, she feel confused and unsure about your intention. During the period you close with her, she was expecting something to happen, but then nothing happen, she started to think a lot, she think "maybe he just treat me as a friend, I should stop talking to him to avoid develop feelings for him." She afraid she will get hurt in the end.
Another possibility is that she at first interest in you, but then she realize you are not the one, then in order to stop giving you false hope, she choose to avoid you. Because she feels if she is not interested in you, she should not keep you by her side anymore, because in the end you might feel that she is just taking advantage on you. She doesn't feel good ignoring you too, she suffers too, because nobody want to lose a friend. If there is a choice, I would not want to lose someone I used to know and become back to strangers.
Give her some time, maybe about about 3-4 weeks later (if you still have feelings for her), then you can find her again and test water, if she still ignore you, then you move on. Some girls got very high ego/pride, even if they like the boy very much, they still tahan not to find the guy. You don't find me, then I also don't find you, then end of story. Ego wins.
Advice Wanted Wat to do A girl start to ignoring you ?