Tell her you will call. Don't have a clue. If you get a clue, pretend you didn't and disregard it. Don't even think about saying it. It's OK if you forget trivial things, like your girlfriend's birthday and eye color.
Ignorance solves problems.
If you can't see them, they can't see you. It is never your duty to take responsibility for your actions. Don't ever notice anything. If you cheat on a girl, but no one finds out, then technically you've done nothing wrong. If you ever find yourself in a position where you have been proven wrong, blame others.