@Jacqueline228 ugh I am sorry. Sounds like this lady was one of my earliest guest who did a "white glove test" for dust, looked behind our washer dryer for evidence of poor cleaning, etc. Sounds like she went through the place-- under cushions, checking items in the fridge, etc. Did you have any inkling she was this way?
I think your response was good to show you are being proactive about the feedback but I might have said something like:
"This is the worst feedback a host can get as every stay offers the risk that a guest could unintentionally bring vermin into our space. We have regular extermination service but I am arranging a visit immediately to determine if any insects this guest saw were bedbugs. I only wish the guest had made this claim during her stay so that we could have resolved the issue on the spot. We take this very seriously and will find out if this is an accurate review ASAP. If so, future guests need not worry-- the space will be fully fumigated. If the guest was mistaken we hope she will be more careful in publicly posting this information for her next hosts. Thank you."