Nothing to do with power. She saw it as a day that the two of you would have together. Now she feels like she would just be tagging along with you and your gaming buddies, and that you'd be spending time with just each other to placate her.Was I wrong? Her attitude seemed childish and somewhat possessive, it's not the first she acts like this and I'm afraid she's trying to gain control over the relationship.
Her going to her mom's with the baby for a week is childish, but I feel like you messed up by not discussing the extra guests with her.