Is she down to basically normal size for an adult female rat? They don't shrink back like that unless they've delivered their litter at least not that I've ever seen. Is she in an enclosure alone or with other rats? I'd remove any adults and go through that bedding looking for evidence that she delivered or pinks were cannabalized or even buried under bedding. There shouldn't be a lot to find as rats don't make much of a mess when they have their babies but I'd still see if I could find any indication she'd recently delivered. Did you ever hear any little baby rat squeeking at all?