Do you feed wet or dry cat food? I know that all the strays I've found have all been dehydrated. My cats also seem to prefer wet cat food over the dry. If she's dehydrated she may being having issues with creating milk.
Another idea is wet kitten food. Its packed full of carbs to help growing kittens but also helps promote weight in adult cats.
Has she seen a vet? The vet might also be able to provide you with suppliments to add to her meals to give her the added nutrients that she'll need. My vet gave me a box of powder that I had to add to my cats meals everyday when I first got her to prevent UTI. I'm sure they have something for preggo cats.