Why does my 6mth puppy hate the new puppy?

My Cavalier KC Spaniel is 6 months old. Today I got a 3 1/2 month old er Spaniel today as a playmate for her and to complete our family but she appears to hate her & be actually afraid of her. She's growling and barking & running away. She has been socialized with another dog and a puppy so I cant understand why she doesn't like this one! Should I leave them alone in the kitchen overnight (this is where the Cav sleeps now) or will this create more issues?