Firstly, where did you get her from?
It may be related to where she was from and the environment that she has been in previously...
She is still a puppy and very easily trainable, but the first thing
you must do is get her out and about to bond with other dogs and humans, she needs training firstly with a puppy class.
All my spaniels have been socialised as early as 8 weeks and
were at their first puppy classes at that age.
This may not be an agressive situation at all but down to how she was treated in her former home as stated.
What you must do is make sure that you are boss over her and be very firm, don't express any fear of her as she will pick up on that which will make her worse in her bahaviour but at the same time she need love and lots of it but at the same time discipline.
I have trained many dogs in the past and a couple of my own have been fiesty but don't give up, I don't believe any dog is agressive, it's the way they are treated.
As for the muzzling, this could make her worse, I would suggest trying the socialising first and flood her with many different dogs and have people approach her in the normal way and stroke her to assure her that everything will be ok, If you do this on a regular basis I believe she will come right, praise her for the all the good things that she does by rewarding her with treats and give her plenty of toys to keep her occupied.
If I was you, take her to a dog trainer but find a really good one that knows what they are talking about.
You must give her a chance as she is so young, she will blossom into a lovely and happy dog, I know I have dealt with them and I know what I am talking about.
I have 5 cockers at the moment (show dogs) and know what training is required for dogs, whether they are showdogs or not they still need the same training and the discipline required is so important.
Please let me know how you get on and if I can help you again in anyway, contact me.
You did'nt say what type of breed she is?
All the best to you