I punched my girlfriend...Don't know what to do!!!?

I was round my girlfriend's house earlier, her parents were out and we were watching a movie. A scene came on where a man cheats on his wife with her sister and my girlfriend says ''you would never cheat on me would you?'' I jokingly said '' yeah i wouldn't mind being with your sister...i'm only with you because she rejected me'' (I was joking, but i did go out with her sister a year ago). Then my GF goes mental and pushes me really hard in the face and starts shouting! I try to explain to her i was joking but she continued to slap and basically punch me in the face and it really starts to annoy me with her not listening and i eventually push her away with my fist and catch her on the nose and blood goes everywhere on the sofa and carpet!! I was so scared so I whistled for a cab and when it came near, the License plate said "Fresh" and had dice in the mirror. If anything I could say this cab was rare, But I thought 'now forget it, yo homes to Bel-Air!'. I pulled up to a house about seven or eight, And I yelled to the cabby "Yo, homes smell you later". I looked at my kingdom, I was finally there, To sit on my throne as the prince of Bel-Air.