I am really reconsidering this friendship. Was I right to get angry at my best friend, or am I out of line?

My best friend and I had plans since last Monday to go see a movie on Sunday afternoon (today). So, I called her today to see when she would come over, and she said that she couldn't, because her parents had made plans for her to go to this mall that has an arcade and a bowling alley. Because the two of us had plans, I had turned down a trip to the museum (which I like) with my parents. My friend didn't even bother to tell her parents that she had plans with me, and now I'm stuck at home by myself with nothing to do, all because my friend didn't tell her parents that she had plans with me. It's not so much that she did this, but that she does it ALL THE TIME! We'll have plans for a week, and then the day we're supposed to meet up, something with her family comes up, and she can't do it. I'm so angry at her. Tell me: am I right to be angry, or am I being out of line? Thanks.