Aww I haven't really experienced this as I am getting ready to add a new girl to our family with Parker (he will be about 6 months as well). But what I think it could be is the transition, they have to get used to one another and her motherly like instinct coming into play. I wouldn't read into it too much and enjoy having the new baby around. I am sure others will suggest too is not take away any rights that she had before the new baby came into the house. Allow her to do the things she always did. Anyways, welcome to YT and congrats on the new baby!!
I am sure you will get some great advise from YT!!