I would just keep her on the same diet, since switching the diet can upset her stomach/give her diarrhea ect...you may need to increase how much you feed her though since she's providing nutrients for more than just herself, but several little pups inside her:)
The best thing to do though, if your concerned, is to call up your local vet and get their opinion.
I'm sure they make special diets for dogs who are pregnant, this is just what i recommend:)
My family breeds purebred labs, and the vet has us feed our prego dogs large breed puppy. Since you have a chihuahua, I'd give her small breed puppy of whatever brand you're already feeding her. Those are the nutrients she needs to give to the puppies.
Also u may want to throw in an uncooked egg every now and again. Just to make sure she has enough protein u can mix it into her dry food.