Ugh. Bunnies are really tough. - My friend's bunny had to be put down last year. He was perfectly fine, and then he wasn't. The vet told us that rabbits actually hide how much pain they are in until right before they are about to pass on. - It's something they do because they are prey animals and they don't want to show any signs of weakness. Her rabbit had some sort of cancerous growth on his liver or stomach or something. We just never knew. He was perfectly normal until he just became unable to move one day (and also became uninterested in eating). She booked an emergency visit the next day, and he was put down that same day. It was the most sudden thing I've ever seen.
Truly wishing you the best.