I was hoping to get a response from you to my request for additional information. It would have been helpful so I could give you a more tailored answer to your question. So I'll have to give you a generalize response and try to cover the various possibilities.
The quickest wa to help a dog that is fearful of humans is to start obedience training with them. I'm not talking about a formal class quite yet, but the humans she knows and somewhat trusts should work with her on a regular basis on obedience.
The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
I know many owners forget that little dogs need obedience training since they often just pick them up and make them do what they want. Obedience training though serves more than one purpose. It does teach a dog obedience commands and it helps establish the humans as the boss and leader. This part is important because it is the boss or leaders job to protect the others in the group which would include the dog. Of course you do need to follow through on that and I'll tell you how you can do that. And finally as she learns that when you say certain words, you expect her to do certain things, she becomes more self confident around people since she will feel like she now understands what you want and will continue to obey to get the treats and be treated good. That self confidence helps her recover from any fear she might have.
Now as the boss, you need to make sure people don't hurt her or even have the opportunity to hurt her. Small dogs can be injured by being stepped on, so you need to ask people to keep their distance. If they want to pet her, they should not lean over her. This is seen as dominance or reven aggressive behavior, so ask them to crouch down and let her come to them. They might have her perform a command and toss her a tasty hot dog sliver as a treat. I do recommend this high value treat as training aids since dogs tend to love them.
Don't let people push her to be friends. Recommend they contrinue to crouch and let her come to them and not to try and pet her at least initially. Let her just sniff them and decide they are not a threat. Don't let people or kids rush up on her either as that can be frightening especially if she has ever been stepped on. So you have to give her time.
You have to slowly socialize her around people. Once you have her listening to commands then go ahead and put her in a group obedience class. That will serve to teach her to listen to commands with distractions but also get her around strangers that she doesn't know in an environment where the owners are not focused on her but on their own dogs. This helps teach her that a lot of people have no interest in her at all and thus she doesn't have to be fearful o them. After this you can take her to parks or outside of school yards. You don't let people interact with her, but stay outside the fences. It is to get her used to the noises of a lot of people and children without forcing her to interact with them. Once she seems comfortable with this, you might try pet friendly stores but still limit her contact with people except to allow them to toss her a treat. You are building up the association in her mind that people don't hurt her and give her tasty treats which should help her become less fearful.
It isn't a quick fix and will take time but usually helps them become a better adjusted more social dog. I do have to warn you though that some dogs that have had a very hard start in life may always have some fear still. You just need to accept that if it turns out to be the case.
One thing I haven't mentioned is that owners can actually encourage the fearful behavior inadvertently by trying to reassure them when they are acting fearful. A dog quickly learns that they might get attention if they act fearful and it ends up becoming a habit for them. So hopefully that hasn't happened in your girl's case.
If you want to give me the responses to my questions, I can see if that changes my response or if there are other things I need to address to help your girl.
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.