My girlfriend pushed our plan to next day. it makes me angry. what do i do with this?

Mate, you need to chillax. She needs to spend time with other people too, and if it's the only time her sister can go shopping with her, you need to understand that. You have to realise that just because you're the boyfriend means you don't come first all the time. She has to make time for other people too. I'm sure you'll see her another time... chill.