I am so very sorry for your loss. She had some underlying condition that was not improving, but it wasn’t presenting itself in a way that the vet was able to diagnose correctly, or she had a relapse that progressed to fast to save her. Please don’t blame yourself, as you made every effort to get her treated and cared for as much as you knew was needed.
Bunnies go downhill very, very fast when they get sick sometimes. It’s just the way it happens with them. The vet can examine her and do a necropsy, which is like a bunny autopsy, to find out what exactly happened. But honestly, I would just tell the vet (or send an email) and copy what you have described her and see what the vet’s opinion is regarding the cause of her death. It is an expensive procedure and cannot help her now. If you had another bunny it might be important in case she had something contageous, but I think we can only speculate at this point.
Just please be reassured that you did what you could. I’m sure your son needs comforting right now, and you do too.
I am going to move your post to the Rainbow Bridge section of the Forums, so the others can know about your loss and offer their respects and opinions as well.
{{{{{{{{{{{{Hugs to you and your family}}}}}}}}}}}}}