That's the problem with pet rabbits. Their instinct is to hide all signs of sickness and pain because in the wild, that means "pick me! I'm an easy target!"...
Sorry for your loss. My kitten died a few months ago. She wasn't doing well the night before so I brought her to the vet. They told me that I could pay $800/night to keep her at the vet so they could run tests or I could put her down. I took her home with me that night. She seemed fine when I woke up in the morning and then she just died in my arms.
Sorry for your loss.