Is this aggressiveness normal?

thanxbaby

DD has been quite a handful lately, tantrums etc...  I know this is normal.  Over the holidays we visited with family that we only see a few times a year (all of our family lives far away) and she was so rough with the babies.  I was shocked because she always loves babies and will try to be a little mother to them, but when visiting with her 16 month old cousin she was awful to him.  We're talking pushing for no reason, shoving toys in his face, and showing no signs of remorse.  All of this when he did nothing to her.  Tonight we were at a party with friends (a very large party with at least 20 kids) she started out being very gentle with a 13 month old but within minutes she started pushing him down for no reason and hitting him all while he did absolutely nothing to her.  I get if he was about to take a toy from her but honestly!

Is this typical?  She is usually very social with kids older than her and her age, but I just don't get where this is coming from?  Both times that it has happened she did seem to be getting tired and maybe overstimulated.