Sounds very fishy... Are they getting information from your vet re: your pet's health? And if so, can your vet provide you with any information or report that he or she gave to your insurer? I'd certainly be making use of my vet to "prove" by your pet's vet records that the conditions were not pre-existing. If you continue to get this kind of treatment, you might have to sue.
Again, even though you are a good client and pay your premiums on time, your insurer is not really on your side when you make a claim. When you do make a claim, you've just become a liability to your insurer.
Having said that, from what I can tell, pet insurance is even worse to deal with than health insurance for humans.