Bitch being aggressive.

I had my 13 month old bitch spayed and since she's been really aggressive to other dogs.She doesn't bother about her sister who visits most days,or my daughters Labrador or next doors 2 staffies,she plays with them all.
She used to be so friendly,dogs we met on her usual walk she used to play with,Now she goes straight for any strange dog,I don't mean a growl or a threatening stance,I mean straight in without hesitation no matter how big or small.I took her to the vets recently and had to take her in another room as she was trying to attack other dogs there.
It's got to the stage where I can no longer let her off the lead and am even thinking of muzzling her when out.I also have a 10 stone rotty doberman cross and she attacks him whenever she gets the chance if the mood takes her,never in the open though,usually from under the table and back under as the big dog can'r fit.She used to be so friendly,it's embarrassing asking owners of erstwhile friends to put their dogs on a lead when we meet them.
It only seems to have started since she was spayed but from what I can gather it shouldn't make a bitch aggressive.It mat be a phase.I hope so but she is getting to be a real pain.

How long as it been, it could take a couple of months for a females hormones to settle which at the moment (if recently spayed) are all over the place and she might be frustrated.
spaying doesn't cause aggression but pain does, have you had her checked? every inch of her, could be anything from earache to her spaying wound, different food, enough exercise?
Your projection that she is going to attack, so you tense up ready will make her attack also, she's wondering why your tense and see's an outlet like the dog approaching.
If there really isn't anything different i would get her back to the vets because something might have gone wrong, causing pain or her hormones out of wack.
If all this is clear there is nothing wrong with muzzling at all, the only down side is she can't defend herself if she is attacked a big dog and it fought back.
sometimes though just knowing your dog is muzzled can lower your anxiety and make the dog react different when approaching other dogs.