Replying to Formal Date

so theres this girl that i really like, and shes on my cross country team and swim team, so we hangout like every day during practice, and we're pretty tight. and shes really attactive and funny and nice and everything. and so she had been showing an interest in me starting probably 3 or 4 weeks ago... so this week, i asked her to the formal dance, which is like a month away, and she said yes. but ever since then, she has been completely different, and shes kinda been a bitch. can anyone help me figure out whats going on in her head, so i can try to get her to like me again... cuz i really want to hook up with her, but like i dont know shes just been different.