Please don’t panic! First off, we’re probably gonna need a little more information.How much did she eat? When did she eat it? How old is she and is SHE a female at all? XD
One time I was eating a chocolate donut on the couch with my little lionhead Ginny and when i looked back found that Ginny had eaten almost half of it without me noticing. Naturally, I panicked and watched her like a hawk for the rest of the week and it turned out she had no symptoms, she was eating and drinking fine and in the months and months since she’s had no problem. Make sure Ivy doesn’t get any more chocolate, only let her have hay and water for a while and watch her carefully. If she stops eating/drinking/pooping, definitely take her to the vet. Chocolate can be toxic to rabbits but in my experience some can handle it. You will probably want to get a second opinion, but for the moment my suggestion is to tell you not to panic and just keep a close eye on your bun. Chances are, she’ll be okay. 