It doesn't sound like she is very respectful of you (or you of her...) However, I would like to concentrate on the fact that you went to a movie the day after you BROKE UP. Did you remind her that you had broken up? Hate to break it to her, she cannot control you, and she cannot expect you to put your life on hold if you two are not together. Now, if you were dating and respected each other, and if movies were your special thing, then you may be willing to forego going to the movies with your friends in favor of her. If she wants nothing else than for you to pine for her, while she spends time with another ex, then she expects something that you should not have to give. It is not like you did something horrendous. You saw a movie- a plain, simple movie.
I know you are upset, but you only get what you accept. Do you want this type of relationship?