I’m so sorry this happened to you and your bunny. Unfortunately, skin reaction at the application site is a possible side effect with any spot-on flea treatment. Advantage and Revolution are generally safe for bunnies. However, each bunny can react differently to different medications. This means they could be fine with one kind, and have a skin reaction to another because the ingredients are different.
Most bunnies though, will not experience a reaction. The risk is small and you just never know if your bunny will be one of the few that will have a reaction until it happens. So please don’t blame yourself. Sometimes it can be hard to identify a skin reaction due to medication vs. a skin reaction to flea bites.
I’m glad that you know now what will work for your bunny. If Revolution does not cause a skin reaction, then it would be good to stick to that.
PSA for anyone who might be reading (but isn’t necessarily a response to the OP) – When selecting a flea treatment for your bunny, never use a product containing fipronil (Frontline, etc.). Fipronil is toxic to bunnies. It can be deadly if applied to your bunny.