It may be something she learned in childhood. Sometimes when people have harsh or overly critical parents, the children tend to mimic that. They set high standards for themselves, constantly compete with other people and feel bad about themselves why they didn't do as good as someone else.

My advice it to try and talk to her about her actions and how it affects you. If she is being mean it may be unintentionals and something that is a norm for her family but hurtful to you.