It could be hibernation or it could be phantom pregnancy. Or, if there's a chance another rat has got to her, it could be a real pregnancy. All you can do is keep an eye on her, keep her cage clean, dry and warm as well as well-supplied with fresh water and clean food, then watch.
From my experience of sick rats, they don't usually go to any effort to make themselves a deathbed. But they Do make themselves nests, either for hibernation or for having pups. While I've never had them hibernate on me (not in Sydney!) I HAVE seen Pygmy Possums asleep (they hibernate with every sleep, every day) and I remember for them, warmth was an issue. Their body temperature would drop when asleep, but if you held them in your hand they would slowly wake up as they warmed up. You could see their skin change from pale pink to a deep rose, and their muscle tone would go from flaccid to being able to grip your finger really tightly with their tail.
I also remember that rats that were REALLY being coddled at my work were kept in a room that was set to about 35C (a smidgin below body heat) with moderately high humidity, also. The Rattus Ritz.
A thought - if she will let you, or if you can observe without having to handle her too much (and upsetting her) then have a look at her skin and/or mucous membranes. If she's a white rat, look at her ears and her tail, also the pads of her feet. The best place to look (apart from pads of feet in a white rat) is inside her mouth, at her gums, but that might upset her. You might be able to get a look at the colour of her tongue when she is drinking from a sipper bottle. If her skin is a healthy pink that is a good sign. if it is very pale, I would consider taking her to the vet.
If she will let you handle her, feel how her paws feel on your hand (in terms of warmth). Or her tail (harder to tell). We used to have a very tiny sphygmomanometer cuff for taking their blood pressure, round the tail. We used to have to lightly anaesthetise them to do this, but ours were lab rats and not pets. We occasionally had some very tame rats that were used to being handled, but even they didn't like their tail being interfered with. The BiPolar (BP) rats - they were being used to study new drugs for treating hypertension. The drugs were given to the rats in their drinking water, the rats lived an otherwise normal life (for a lab rat) in terms of having a comfortable cage, plenty of food and water and room service. They were left to eventually die of natural causes, since the study was also assessing any possible long-term problems.
I hope you get some good answers for her.
Marg
