It seems she’s having stomach trouble. If you have simethicone baby gas drops, give her 1 ml of that for starters. When a bun gets tummy trouble, gas will ensue as a consequence, and that is very painful for a bun. Gas drops help the gas escape. You can help by elevating her butt slightly, after you’ve given the gas drops. The fact that her behind is flattened could mean that she is pressing her belly to relieve pain.
Also make sure she isn’t cold. A rough estimate can be had by feeling her ears, if they are cold, give her a warm water bottle or handwarmers wrapped in a blanket (you can make a quick heat pad by microwaving a sock filled with rice together with a glass of water to spare the microwave). She must be able to move away from the heat source if she doesn’t want it. If she will accept sitting in your lap, your body heat will work as a heat pad to warm her in a safe way.
I do think this is a case where you need a vet. Rabbits often need fluids sub q and a gut stimulant plus painkillers to get the tummy to start up again and appetite return. Don’t wait too long to contact a vet, time is very much of the essence here.