I'm going to tell my girlfriend that I don't want to pick her up
Why are you going to tell your girlfriend that?
I am mad at her and want alone time.
If you are angry with her you cannot just strand her somewhere. You should tell her you are angry or upset if you have a problem.
Why should I tell her? She knows if I don't pick her up that this is my way of showing I'm mad at her.
This is not a productive way to do this because you promised to do so.  Use the time picking her up to have a talk with her about what is upsetting you.