I honor you for rescuing this four-footed baby. Your main resource right now is your vet. Ask right away about feeding. The vet needs to check her thoroughly to see if she has any internal problems, so you want to contact the office today. (I just noticed that you posted this yesterday, so I hope you've already made the call.)
Here's your secondary resource. Every breed of dog has a rescue organization these days, so go online and find your nearest Chihuahua rescue group. You'll be able to communicate with a number of people who have already had the challenges of healing and raising a rescued dog, and I dare say you'll find it a lot of help.
You can't blame the puppy for being frightened. She doesn't know what awful thing is going to happen next. You'll need to help her realize that she is safe and secure with you. It will take some time, and she may do a little testing - she's a puppy, after all - but that's all right. Along with that, you need to show her that you're the leader (dogs are pack animals, remember), so she will know she can depend on you.
If we could send Mamapedia "flowers" to posters instead of answerers, I'd send you a bouquet of those purple things!