
Some kids are just like that, just in the same way that some adults are like that.
I think the technical term is 'sensitive'.
My DS loses it when he has to interact with more than one kid at once and he can get very stressed when we are at the supermarket and it is busy.
He is easily overwhelmed.
It may just be a personality trait.
Have you tried talking to her about it?
She may have a genuine fear of other people.
There is also something I heard about ages ago called a selective mute. Meaning that in social situations or around anybody other than close family they freeze and CAN'T talk.
Like I said.. Maybe talk to her about it and ask her how she FEELS when it happens.