Weight loss and drooping ear

[sandycandle]

My 3 year old rabbit has suddenly lost a lot of weight. We have taken her to the vet who gave us some critical care food and told us to keep an eye on her. She is lethargic and now her right ear is drooping, which appears to be causing her to lose her balance. She is eating and drinking well and her poo is normal, which I think are very positive signs. Over the last 24 hours she has put on 100 grams but she really doesn't seem to be getting better. Any ideas on what might be causing this and how we might best treat her?