Why are some girls just so mean?

I went to meet this one girl my boyfriend's friend was dating and she seemed really nice and such. But when i left the room all i heard her doing was making fun of how perky I was. She tried to sound like me and seriously we are all in our 20's! She doesnt even know me. What is the deal? I thought the high school years were over. Yeah i shouldnt let it get to me but goodness, it hurts.