Abz, maybe you didn't get an invite cos your boss knew you were working... men can be like that sometimes: "well there's no point asking cos I KNOW she can't come anyway". I think they're wired differently to women. (a personal opinion - no offence intended to any men out there). I'd just let this one drop. If you're not invited to the next one when you ARE available then is the time to be annoyed.
Hope everyone had a lovely day
~Silence~