I would think that your kitty just doesn't like its tummy scratched. Many
kitties don't.

DES

Save 1 life and you're a hero.
Save 230 million and you're a Starfleet officer.

remove "nospam" from my addy to reply.

Being allowed to touch a cat's tummy should be considered a priviledge
and a sign of total trust, something that should never be taken for
granted with cats. When cats show each other their bellies it is not
necessarily a friendly sign - lying on their sides or their backs they
can use all 4 paws with all 18 claws and their teeth to attack or defend
themselves. Cats can learn that humans mean no harm and that getting a
belly rub can be a pleasant experience - sometimes they will even ask
for one themselves, but this cannot be forced.
Camilla

Possibly:

-The cat doesn't like you.
-The cat doesn't like it's belly rubbed (sa others said, not universal)
-It's got a full belly.
-It's being psychologically defensive, since this is actually not a "trust"
thing at all, it's actually a power play by the cat/dog to have you rub its
belly.

I never scratch a cat belly that's offered to me, only the head or the back.
When they really want it, they stretch themselves out long and are sleepy
and purry.

My dog friend thinks cats are vicious because once one attacked her like
that. They are not like dogs who enjoy a belly rub.

Carola