It sounds like just an isolated case where she got startled or something. I still agree that its time for socialization on a scale for an 8 week old pup without full vaccinations. I wouldn't worry about this one instance of growling then. My aunt has two chihuahuas one a total sweetie and one thats vicious toward strangers and kids. They both were raised the same. I'd never say that figures for a chihuahua.