I'm  not going to take full responsibility for my girlfriend's dog getting out.
What is your reason for not wanting to take responsibility for her dog getting out of the house?
I had literally only opened the door for a second. The dog should have been on the leash.
It is okay to make mistakes. I think you should acknowledge that it was partially your fault. Maybe talking to your girlfriend about it might help.
Wtf, she should've watched the dog.
It sounds like you both made a mistake in this situation. Instead of blaming her and being angry, you should try and comfort her and tell her it was partly your fault as well. Next step is to go and get the dog.