Having her spayed may calm her down a bit, is she due a season, perhaps that is why she is a bit aggressive at the moment, is the weather hot where you are? dogs can get nippy if they are hot, I personally wouldn't use a muzzle as it can make matters worse, she will feel really vulnerable around other dogs and may become more aggressive because of it, I would be more inclined to walk her on an extendable lead when there are people and dogs about, shorten the lead if you feel there could be problems, start socialising her again with others bit by bit, try taking her to obedience classes, this will socialise as well as you doing a bit more training with her.