My gf wants me to go out with her and her friend tonight but I don’t want to go.

I spent most of my savings to help out a few months back when my sisters fiancé died and I’m trying to save money again. My car isn’t paid off so I have to pay it every month along with rent. and i just spent 1,400 this week on rent and bills and I have to pay my car next week. She still lives at home with her parents. I just don't think she understands at am I being an asshole?