Is this a good friend or not?

i have this friend and her and I have been friends this whole school year until we went to the movies with her friend Lizzy from her old school and now Lizzy and I are like best friends and my friend Amie has been so mean to me but then she turns around and is so nice to me and she acts like my best friend and now whenever i bring up the name Lizzy she gets really mad at me when her and Lizzy dont even talk or hang out anymore. What should I do?