I can't talk to my friend.. it's so awkward

I think I've got the ick..
So there are 3 of us in a group and me and another girl in the group are close and we've noticed recently that our other friend is being a bit more distant.. and it's a bit strange. So she never starts a conversation, if I buy an item of clothing the next week, what do you know, she's just gone out and bought the same item and is wearing it and don't even make it natural and say "ooo I like that, I might get it too". She copies what I say literally straight after I say it. And it's come to the point where everything she does just annoys me and the other friend and when it's just me and her we just can't talk and if we do it's awkward forced conversations that are just boring.
Opinions?? What do I do??

Perhaps they are trying to fit in with what you guys do and wear because she feels left out and wants to be the one whose close? no offence mate but it sounds petty to me, I get mates can piss you off sometimes but just be real with them and ask them about your problem with their behaviour.