Jack Russells are very dominant with other female dogs. We've had one. It was not a good mix with our little fox terrier. The first thing I'd do because of her age would be to have a vet check her over. If she's o.k. then...are you chuckling or laughing when you "show your teeth"? If you aren't then she takes it as an unfamiliar or not normal look on your face and it frightens her. (My take on it) Also some dogs don't like a confrontational look. I've a tiny long haired Chihuahua who will turn and run if I make eye contact. He was a rescue too. I have to be careful not to look him directly in the eye when I call him. For some strange reason, it frightens him. ......'Course, it frightens my husband too.
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Hope things straighten out.
Billye