Y is My dog is aggressive to other dogs now that my other dog died?

My oldest dog was 15 and I came home the other day and he had passed my other dog who is 3 and had grown up with this dog in her life was very depressed and kept looking for him... She has become very attached to me to the point she even fallows me into th bathroom and when me or anyone takes her on walks she's is really aggressive to other dogs and that's never happens she's always been really friendly to other dogs why is she doing this?