Ugg, Girlfriend didn't really respect me.?

So you know how you make plans on monday for friday. Well you know when you already have plans and someone asks you if you want to do something that goes against those plans you would automatically say that you already have plans. Guess what my girlfriend did... cancelled our date for tonight to go see a movie with her cousins who then decided not to go to the movies and rented movies instead. I have already talked to her some about it but I have stopped being a nice guy about it and started demanding when we have dates and making sure she doesn't have any plans on the days when plan ****. What else should I do about this?