An update to this: got in touch with the rescue coordinator and took pup to the vet. Suspected parvo, as there'd been a pup with parvo in the same shelter (isolated, but still). Luckily she came up clean for parvo but 3 days later one of her littermates died from distemper.
Never did figure out what caused my pup to be sick, but after a week on meds she perked up and was back to her normal self. No signs of distemper either, thank goodness.