Why is my German Shepherd aggressive when other dogs come near me?

I have two female German Shepherds. One of them is about a year and a half old and the other is four months old. They are both great dogs and get along well when they are together. The four month old follows me where ever I go. If I sit down somewhere she will sit on my lap or right next to me. She has... show more I have two female German Shepherds. One of them is about a year and a half old and the other is four months old. They are both great dogs and get along well when they are together. The four month old follows me where ever I go. If I sit down somewhere she will sit on my lap or right next to me. She has recently started growling and barking when people or other dogs come near me. She has now started getting into fights with other dogs if they try and come near me and I am afraid it might escalate to her biting a person. I am worried that if she continues this behavior I will have to get rid of her, which I would absolutely hate to do. Why does she get so aggressive when others are around? How can I correct this behavior?
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