Girlfriend is uncomfortable with me going?

This girl invited me to her birthday party...she’s extremely flirty with me and I am not interested at all but she is always making flirty comments with me. My girlfriend is understandably uncomfortable with this but said she trusted me and didn’t care if I went. I wasn’t gonna go but then my friend asked me if I would go with him just so he doesn’t feel alone there. He has been going through some rough things and so I didn’t want to say no so I agreed to go with him. I told my girlfriend I was going just so she was aware and then she told me that it made her really uncomfortable that I was going. She is basically asking me to not go but I’ve already told my friend and the girl who’s having the party that I’m coming. What should I do?