In our experience, it is just the way life is. I think this is a big reason so many kids around the age of 12ish start narrowing their activities, friends etc. They tend to to identify and hang out more in small groups; the days of play-dates or just putting kids together and them getting along well are left behind.
Girls can seem or be mean, clicky, or just more into their own feelings than the ignored or excluded one. It doesn't mean anything about your dd other than she isn't one of their group.
She sounds like a sweetheart, and these growing up lessons can be very hard on sensitive kids.