>_<
Just be honest and tell her that you're not going to lie, you feel uncomfortable around those people that she's around and when you're around them you feel as if you're not even there because you feel like no ones noticing you and like you're in a room full of strangers. That's all you have to say, there's nothing to it and you definitely won't have to lie.
And also, you shouldn't feel like a "douche", I mean you have your reasons BUT I don't think you should let your reasons take advantage of you every time. Next time you should go regardless of how you THINK it will be, you didn't even try to go to see what would happen. If you don't think people will talk you and you think they'll leave you out of stuff then join in yourself. It'sa new day, another time..next time at least try to go instead of making assumptions.
Tell her the truth: you feel awkward around her friends. If she's your best friend, she'll understand.
yeah. if she knows you as a best friend. she will understand how you feel