Claim Rejected for pre-existing condition that I didn't know about.

We felt a bump on my 4 month old kitten's belly and took him to the vet to check it out and the vet diagnosed it as an umbilical hernia. Now my claim got rejected because it was deemed a pre-existing condition. We got this kitten as a rescue and 8 weeks old. He had been taken to the vet for a check up and his vaccinations and the vet said he was fine. So I want to know is how is this a pre-existing condition if I didn't know about it before I took the policy out and two vets didn't pick it up. My first claim I did was quick and efficient where my second one just boggles my brain. Really dissapointed.