I'm so sorry. It may be helpful to have a necropsy done because although there have been some other opinions posted in the thread about what it could be, it looks like poisoning to me. When my rabbit had a bad reaction to ivermectin he did the head down position seen in the first photo. In order for the rabbit to die that quickly, it would have had to be something traumatic/acute, like a spinal injury as suggested by Pipp or poisoning. A lot of things in the house are poisonous to rabbits and you may want to assess what she could have gotten into in order to make your house safer for another bunny.
Thank you all for your help and consolation.
