I'm mad at my girlfriend for handing the tip to the waiter
Is there a reason you didn't want her to give him the tip?
I went to eat with her, she could have managed that better without leaving me alone.
It's wrong to be mad at your girlfriend for something innocent. She probably just wanted to make sure that he got the tip, and to thank him. It is nothing to hold against her
It's just petty tbh. She's a bitch.
I think it's the whole situation that is petty, not your girlfriend's behavior.  It is wrong to call her a bitch, especially since she did nothing wrong here, and was actually being very polite and helpful.  She sounds like a great person, and I hope you can recognize that.