I just adopted a very sweet female shepherd mix. When I got her I did a meet and greet with my other dog a Chihuahua x and all went well. However, when I introduced her to my roommates dog a 7 month old shepherd mix she became very aggressive not playfully nipping at him but growling and biting. I'm so confused her dog loves other dogs and made no aggressive moves. I don't know if it is that he is bigger than she is or that he's a male or what but if it isn't sorted out soon, I will unfortunately have to return her. Please any help on the subject would be great.