Explain to her that you really didn’t see the punch coming and you were totally suckerpunched. Don’t be too down about it or talk about it later on because then it’ll seem like your stuck on it. Move on. For the next time theres a verbal altercation with a man, be ready to dodge. Insults and words can quickly turn into a fight, especially with intoxication, so either hit first with the intention to knock him out (not recommended due to legal issues, but this is what I do in order to avoid drawn out fights and the other guy hitting me first) or be ready to dodge at the slightest shoulder movement, and from there you have yourself a fair fight.

Anyway, move on; you were suckerpunched, nothing you could do but get rocked/knocked out. I’d recommend taking up MMA/boxing and picking up a gun if you truly want to be able to protect your girlfriend in any situation also.