I have a 16 month shih tzu she is so love able of course if you're a stranger she must smell you before u can pet her cause if not she has her tail between her legs and hides between mines. My problem with her is whenever a dog comes close she doesn't mind it but when their sniffing her especially her bum she growls then tried to snap. How can I stop her from doing that. So far it was female dogs

You can't stop a natural instinct. My guess is she is not spayed. Correct? She will grow out of this response, the more she is socialized, but have her spayed.

It sounds like she hasn't been properly socialized with other dogs. Is she spayed? That can play a huge part.

She needs to get out more, meet dogs. Growling is a natural way for a dog to let another dog know that they're not happy with what's going on, and to back off. But sniffing each other is natural for dogs, it's a way to get to know each other. Growling because she's being sniffed means she really doesn't know how to be a dog, probably because she hasn't been given the chance to learn.

She allows them to sniff her in her face and stuff but when they get to her behind that's when she doesn't like it. But she can walk by the dog or the dog could be sitting next to her but when they do that she growls and tries to snap which she listens when I tell her stop or no