Puppy barking and growling out of the blue?

Hello,

I have two cavalier king charles puppies of 5 months old. Up until two weeks before xmas, we were taking our puppies out every day on their leashes and eventhough they were a little difficult to walk as they are still in training, they were fine around strangers, other dogs and in general they behaved pretty well. However, all of a sudden one of them started exhibiting barking and growling behaviour around dogs, people, noise whilst she was on her walks. It happened out of the blue, I don't remember there being any specific event that could have caused it, i.e. no attack or scary encounter with another dog or person. They have both been pretty well socialised as they go out all the time, meet other dogs and people in a safe way on a regular basis. Is there something I'm missing? And what can I do to help her? I think that she's scared and showing us by growling and barking but how can we help with this? Even removing her from the situation doesn't seem to help, she continues to growl for another few minutes after we've removed her from the situation. We try and calm her, reassure her and reward her good behaviour. What do you suggest?

Thanks!