What you are allowed to do depends on your status; the higher your position the more rules you’re permitted to break.
You do know that if the tables were turned, and she were getting off her lazy arse and having a "get together", she'd be doing the same thing as you and no one would be saying a word because she is a ministerial servant-ette.
BTW: Was she going to be invited? Maybe she was just pissed you were excluding her, and for good reason.