Why is she acting like this when I'm treating her the best?

So It's my BESTEST friend birthday tomorrow and all day I've been talking about it because I'm so excited I don't know why it's just that's shes my best friend and she knows I'm really excited. I posted a video saying happy birthday on her facebook wall and I have been talking about it all day and at 12 midnight I'm gonna give her the best happy birthday ever and she knows that but she doesn't seem too excited like shes giving me one worded responses and like she acts like she doesn't care idk I always come up with assumptions but she's being really weird. what should I do?