I suggest you get her back to the vet and get the ear checked out again, the aggression may have been linked to her feeling pain, discomfort or being wary of the ear being touched.

As you don't know her background proceed with extreme caution.

Get to training classes where someone can help you with her behaviour on the lead - this is also, usually a fear issue. You need to be super confident in how to handle the situation when she shows aggression as it seems to be the lead that is the issue here. This is restricting the dog's options when faced with another dog - the option of running away is lost for her and so she responds by threatening. The dog's "critical area" is greatly increased. This is the area that the dog feels comfortable letting other dogs come close to her before she begins to think of her defence options should things turn nasty (it also applies to people, cars, sticks...almost anything for some dogs). A lot of slow, calm lead work is required - pretty hard work for you BUT this is a highly changeable situation. Get some help with it and things will almost certainly improve. If you have a behaviourist in your area that you have heard good things about - ideal.

Don't panic, you have a lot of scope to improve the situation and are obviously willing to try. Good luck, let us know what you do and how things turn out.