Rule #1: Don't tell your girlfriend shit.
Do you tell her when you cheat on her?
Do you tell her you're thinking about some other chick when you're banging her?
Does she know that you think your last girlfriend had nicer tits and gave better blowjobs?
Did you mention anything about that hooker?
Do you inform her of what medication you bought at WalMart for that horrible bout with flatulence you've been having?
You can tell her anything you want, but don't be surprised if it comes back to haunt you. There are some things she's better off not knowing, and your AS usage is one of them, unless you're really sure about the strength of your relationship and her maturity and knowledge level. But even then...