Growling at other Dogs - PLEASE HELP!!!!!

Hi
Can any of you shed some light or try and help me on this...
I have had my 15month old girl from 8 weeks old. She lives with a cat and a Cavalier King Charles Spaniel who is 9years old. She has always been submissive with other dogs and if she meets any when out on the lead she lays down and rolls onto her back.
Her and my cavalier get on like a house on fire and there has never been so much as a grumble from either one of them EVER.
She has always been well socialised and meets other dogs on a regular basis. She has never shown any signs of agression and has always acted like a little cat rather than a dog.
Until yesterday.. I was out walking her in the snow, she was running in fields off the lead for 30 mins, I then clipped her back onto her lead and carried on on our usual walk. She was on the lead and a tiny Jack Russell came upto her (also on a lead) and was barking and growling at her. Usually she would lay down and roll ovr but she growled nastily back!! I kept walking, 5 minutes later we passed a minature poodle who was just simply walking buy and did not growl or bark at her and she decided to bark at that also. So I told her off... she hates to be told off and sulked for the duration of the rest of our walk. As we were almost home we passed another Staffy, I let her go upto it and she was fine (no growling) I really cannot work this out and obviously I am worried that she could be turning nasty. She has not been set upon or picked on before this happened and it seems to have happened over night. Also she has just found her bark and has started this last week to bark and grumble at things on the TV and noises outside.
Is she just going through a teenage faze or could she potentially becoming nasty?
Please help!!
p.s she had her second season in December so she is not in season.

Logan has began to bark at some dogs (only Alsatians/Huskys). He was attacked by an Alsatian when he was 11-12 weeks old. He was nervous and scared of other dogs like these as opposed to being submissive up until a few months back. Now he is very defensive towards them. He will bark and growl. It is more of a nervous thing than an aggression issue.

The way I deal with it is when I see a dog approaching me and see Logan tensing up I ask that the dog is put on lead and stand at the side of the path and let them pass me. Then I try to keep Logan's attention on me and praise him for good behaviour once the dog has passed.

Some dogs just don't get on with certain dogs. Rosie is maybe just not a big fan of little dogs. I wouldn't panic that she is getting nasty etc, but I would try and manage her issues now as they have just started and will be easier to deal with.

Thank you very mcuh for the advise, I have been worrying.