My dog has started growling aggressively?

quite recently a friend stroked our dog quite vigorously at the rear of her back, i don't know if this has triggered this but she start acting differently almost instantly, since this has happened she has started growling aggressively towards my children and some other friends, she has also started to collect small toys in her bed, she has gone from a happy loving dog to a cowering strange dog within about 10 days please help as the dog she is at the moment is one we don't like. She is a 1yr old jack russell, i haven't smacked or shouted i have done the normal thing by telling her she's a bad dog an then ignoring her an she goes out for walks every day and the children use to play with her daily as well.