I had a mother do this to me in year 4, at lunchtime.
It was completely out of line and all I had actually done was to tell her daughter that I don't want to play with her because she was bullying me (it was an ongoing thing and the mum just couldn't see her daughter in the wrong). After that I was too scared to stand up for myself and I was petrified of her mum doing it again. It was made worse by the fact that the school didn't see an issue with it.
Regardless of how this girl is acting, it's completely inappropriate for you to approach her directly, in school.