Diana didn't register the pain at first as the shock dulled it. Once she did begin to feel it, her anger took over in response to what happened and she got ready to launch herself at Celeste. She would show that little human what happened when someone forgot where their place was.
She only managed to land one slap on the small human before someone intervened to break it up before anything had gotten worse. It didn't take much to pull Diana off and away from Celeste.
"Alright, alright. That's enough for now. I don't want the two of you doing this. You both should know better." Edgar decided to intervene at that moment in time. He didn't want to have to deal with breaking up a serious fight if things escalated to that point.
If the manager hadn't chosen that point in time to get involved, Pepper was ready to take it upon herself to break them up before the other had a chance to hurt her friend.
