You have not mentioned at which stage of pregnancy your chihuahua is? Loss of appetite is common at the start of pregnancy and at the time of labor in dogs. Loss of appetite other than these 2 occasions is not normal and can be dangerous for mother and puppies. Dogs can stop eating in response to any diseases. In my opinion, you should take your dog to vet and give him complete medical history to diagnose her problem. Otherwise, it will not be good for your dog.
Yes, have you had pregnancy definitively confirmed by a vet? How far into pregnancy do you think she is? Dogs can get a serious uterine infection called pyometra. They also have very difficult pregnancy's and births and can need a lot of help from their vet's. They also tend to have serious specific vitamin deficiency issues during pregnancy and after. All can potentially result in the death of a female so please have her checked out.