Usually my dog is fine with strangers we've had a few friends in the home and outside, and she got excited, and didn't show any signs of fear, but today when i took her for a walk a very nice stranger asked to pet her, she backed up pretty far and looked terrified, this seems really strange because she's a jack russell breed and they're usually not shy at all, we got her fairly young so i dont think she was abused before, any way i can help her? and why is she ok with some people and not others?