I am 100% sure she didn't get pregnant here (all my boys are neutered and she hasn't left the house, not even an accidental escape). However, the kittens were 3-4 weeks old when she came here, so it's entirely possible she was already pregnant when I got her. I took her to the shelter yesterday (the vet tech who works there did me a favor) and she said she couldn't say for sure if she was pregnant when she felt her belly.
She is eating constantly, but so do most of the nursing queens I've fostered, and since she already had the kittens when I got her, I can't say for sure that she's eating more, less, or the same as usual (though judging by how skinny she is I'd say she's getting fed considerably better here than she was at her old home).
My plan was to pull the kittens off her this weekend, but I can do it earlier. I'm trying to balance the needs of the kittens (who are small and need all the nutrients they can get) with the needs of the mother (who is skinny and in need of proper care herself) and with the needs of her potentially unborn kittens (mama didn't start to "show" until about 5-6 days ago, so I haven't been treating her like she was pregnant. I assumed she wasn't, given her poor body condition when I got her and the fact that she's already feeding 6 kittens).
It sounds like the general consensus is that she's pregnant.
Poor mama kitty. In that case, the needs of mom and unborn babies outweigh the needs of the older kittens, so I'll pull her off them tonight and get her packed with as much kitten food as she'll eat. If she's not pregnant, no harm done, she'll just be a little healthier than she is now.