Aggresion Towards Puppy

by Kim
(Ontario)

My mother's Aussie, Clare, is three years old and grew up with my parents on their cattle farm. She loves people, and is generally well behaved, but when I bring my 4 month old Pug puppy over to visit, she acts like a completely different dog! She is obsessed with him from the moment he enters the house - she pants heavily over him, doesn't let her eyes leave him for a second, she drools over him, and appears very tense and high strung. At any sudden movement on the puppy's part, she snaps at him without warning, and has mouthed him a few times. Because of our constant vigilance she hasn't bit him yet but I fear that she will. She acts like an absolute tyrant when I bring my puppy over. I don't understand if this behavior is aggressive/dominance, jealousy, an uncertainty of how to deal with him, the innate desire to herd him, a lack of socialization as a puppy, or a combination of all of these things. I haven't had any luck searching the web trying to identify this behavior. Has anyone experienced anything similar to this?
Thanks!