I have a one year old American pit bull terrier who just had puppies; now she is being aggressive. Why? ?

My girlfriend has been around her plenty of times but does not come around really often. Today when I was showing her the puppies Sadie ( my pit) was just fine then suddenly snarled and snapped at my gf! So o put her up and just assumed she was being overly protective of her pups. I did correct her actions with a firm yell and smack to her snout. So around a hour later I let Sadie out to go to the bathroom and when I let her back in she immediately locked eyes with my gf and began growling intensely bristled her hair and fake charged! She has never offered to do that with me or my brother but has been overly aggressive lately. I realize it's natural for dogs to be protective of their litter but she's really worrying me. Theirs kids next door ect.... I just don't know what to do. I'm thinking about letting her wean these pups and get rid of her; I'm just not willing to take a chance.