Just ask her, save a lot of worry man
The way you phrased what's going on makes it sound like her friends have passed judgement on you and deemed you unworthy and your friend cares what they think. It could be anything though. Maybe she is busy and seems that way. I have a feeling she doesn't feel the same way you do.
1 little word.... jealousy
3 little words she wants your attention back
i bet her friends noticed and pointed out to her that, hey- you like her. so now she knows (maybe she didn't, or only suspected, before), and she's..digesting the information. rotcod made a good point that could be a part of it, too.
Were you very drunk ?
propose her now, she's in love with you..
People sometimes act differently around different circles of people. I have known one friend who broke down in tears because she didn't no how to act or what to do when her different circles of friends finally met. Your friend might just not know if you think differently of her, or not how to process the "different worlds" colliding. I am not positive that is what is happening, but you can always just ask her. Let her know you had fun, and you still love her, and just want to know if everything is okay.
Well a few things come to mind what could be the issue.
Unfortunatly the first one that comes to mind is that her friends do not like you. Even if you are her best friend, better than the rest, they can still push you away from her.
The second one I thought of is maybe she loves you back and her friends made her realize that, or you meeting her friends and them liking you made her realize how much she liked you too. Now I dont know your best friend, but in my situation my best friend would push away, and the only thing to do is keep pulling.
Im sure there are a few more reasons but the two most common and well serious ones would be her friends pulling her away from you, or her pushing you away. Or possibily both.
I wish you luck.. :)