I mentioned before that my son was bitten by a spider and it got infected. The doctor said that it didn't have to be a dangerous spider, but that all spiders have really nasty mouths. Within 24 hours it was hugely swollen, painful, hot, and very very red. She outlined it and told us that if it spread he needed to go to the emergency room. She said that it didn't have the indications of a black widow bite or a brown recluse bite, which are the dangerous spiders in our area. My point is that even run of the mill everyday spiders that we wouldn't think are dangerous can still cause nasty nasty problems if they bite.
I'm very sorry about your bunny. It's never easy to lose a critter.