Based on what you have said so far, AZ...I can only tell you this:

Your girlfriend is doing what alot of people do in this kind of situation.
She is trying to make you hate her, because she is going with your mate - it's an easy option (for her).
The plan goes like this:
She'll start getting 'tetchy' with you - kinda frustrated and narky about the smallest little issue (check your memory dude, it's probably been happing for about a week).
Then she'll just come right out and say summat like, "You're a complete dick-head!" - already, you are off-balance, trying to think what it is that makes you a dick-head, right?...this enables her to hide her guilt better, 'cos you're all messed-up, trying to think about what makes you a dick-head, and don't see the signs.
Now, when you ring her, she's at your mate's house, and says that your mate don't wanna talk to you - she is preparing the transitional stage, from you, to him.

A glimpse at the future, my friend...
She'll speak to you soon enough, and when she does, she'll probably tell you that she's had enough of your 'dick-headedness' (what ever that means ), and that your mate doesn't act like one...or summat like that.
Then, you'll spend the next week or so, sitting alone in a room, with your head in your hands, wondering what the fuck you did wrong...

I speak as someone who knows this senario, dude.
My advice:
Confront her, face-to-face (if possible), KEEP VERY CALM, and ask her straight.
No matter what the answer is, KEEP VERY CALM (it's tough, I know , but you'll feel better for it...trust me).

The truth may hurt....alot.
But, at least you'll have the truth.

This thing may be completely innocent and clean...but you'll be better able to deal with the truth, than you will be able to deal with the paranioa - believe me, dude, I've been there, and it aint pretty

I truly hope that things work out for you, and that this is just another one of those silly 'girly things' that girls do.


Res...