Why is my dog acting sad?

Fleas

Aggression shouldn't be a fleeting thought of household life and aggression should NOT BE TOLERATED. Aggression shows instability within the household and aggression can happen for a number of reasons.

Sudden aggression often has ties to neurological disorders and nervous disorders, as well as other behavioral changes. If I were you I'd take the dog to a Vet, just to be on the safe side.

in the experience that your canines is performing completely diverse from the way she usually is then there is frequently some thing only no longer remarkable. Is she eating ok? Does she circulate around usually? Is she going out to do her corporation and in checking that does each thing seem commonly used. No blood or mucus in her poop. If she maintains to act that way, examine alongside with your vet.

pet her give her lots of love, that should cheer her up.

what is going through her head could be anything, she could be thinking that she is getting less attention she may feel bad about the fight...., it could be loads of anything... but my advice is to give her lots of love and go play with her. All dogs love to play

give them a ball to chew on