I'm going to fake a narcolepsy attack.
Why would you fake something serious like that?
So I can throw a party at my girlfriend's place instead
That is a terrible thing to do. You shouldn't fake medical disorders. If you want a party then you should ask your girlfriend about it instead of trying to fake something.
She would probably see through my ruse anyways.
All the more reason to tell her the truth. She won't like you trying to trick her, and faking disorders is bad already.