Maybe ask her if something's wrong. If she's suddenly changed her attitude then something must have happened in her life that's causing her to be like this.

If she's really your friend, telling her about it wouldn't affect your friendship one bit

When you tell her, be kind and loving. There's this technique that might help called a praise sandwich (I think) and you do it like this:
1. Something you like/appreciate about them
2. The critique or thing that's wrong
3. Another thing you like or appreciate about them.
It can help the person being critiqued or corrected to not feel attacked and to feel you love them.

I agree with micky. I don't think you should outright say that you don't like her attitude, because that could really offend her (yes, even best friends can get insulted when you criticize them. It's not like being their friend is going to really make it so you're immune to them feeling insulted). But anyways, ask her if someone is wrong and all that, and then leave it at that, I think. If you're still being annoyed or hating it, maybe start distancing yourself. Sometimes it's hard for someone to change their attitude, so you'll just have to live with it.

And Adam, as I said, being their friend doesn't make it less offensive, really. Friends being criticized by friends can really hurt. Sometimes more so than if it were only said by an acquaintance.