lol you actually made me laugh... well I'm a girl and I can tell you at least in my experience it's completely normal. It gets hard because because blood is trapped in there when you're excited but as soon as you ejaculate the blood is released so it goes down. All you have to do with your girlfriend is to try hold off coming too soon lol. Good luck

Yes, that's normal. Men usually lose their erection after they ejaculate, although some men do stay erect for longer.