Behaviour of a mother cat with 4 kittens

My cat, who's just had 4 kittens, used to be so lovely with us and my 8-month-old little Shih Tzu. She had her litter in our bed and actually likes the dog giving the kittens little licks; she even grooms the dog herself. However, as soon as she’s in the garden, she hisses and attacks the dog. We're at a complete loss and don't understand this change in behaviour. She’s affectionate upstairs, but gets really defensive downstairs and in the garden the moment she sees the dog. Why??? Is she going to try and drive the dog out to protect her territory? We don't want to be forced to choose between them! Everything was going so well. I’d really appreciate some advice from a behaviourist, please. Help me, fellow cat lovers!