How to cope with a stupid friend?

it was my choir's concert yesterday and my friend changed to her uniform during our break and after she changed she was like am i fat? and she friggin took me beside her not knowing what's happening and then said, nope im not fat. i'm not skinny, just average sized but i'm pissed cuz she made me feel bad about myself and im WAY taller than her. other times was like she would blame things on me and tell them to my mom and stupid enough, MY MOM believes her and not believes me! my mom and her mom are friends so yea. like i really don't like how she treats me and im 2 years older than her. what should i do? if i tell her than she's going to tell me that i'm a liar and my mom won't help at all.