I'm mad at my best friend because she canceled a sleepover
Why did her canceling the sleepover make you mad? Maybe she had something that came up expectantly.
I don't know, I just really enjoy spending time with her.
It's normal to be upset when someone cancels plans, especially if you were really looking forward to them. While it's OK to be upset, you should also try to be understanding and empathetic. If you get angry with your friend for canceling plans, then they may be hesitant to make plans with you in the future.
I know, I won't get mad at her.
Yeah, sometimes stuff comes up and makes a mess of plans. It's cool that you have this understanding. I am sure she appreciated it too.