What does it mean that my girlfriend "Isn't in the mood to hang out with me"?

It might be possible that she is undergoing some mood changes due to her periods or anything. What you need to do is know where you stand in this relationship. Yes, she must have said yes for that hanging out party or mother meet and should not have said NO. Giving NO CHANCE to this relationship might be a mistake. Let time decide it for you. Do not call her and after you leave do not be in touch with her through mails and calls. When you come back her hug will give you the answer whether she is really in love with you!