This one can be tricky -- and harmful if not played right! On the one hand, your cat could be in a happy, playful mood, content to let you reach out and give her a tummy rub. "Cats can learn to enjoy belly rubs. Of course a lot of cats don't, but if you start out doing this when they're young kittens, they can learn to enjoy it,".
On the other hand the belly is a vulnerable area to your cat, and she's probably protective of it. A cat that is upset or feels feisty, for example, might roll onto her back so she can unleash her most powerful weapon -- her back claws. So if it's playful fighting your cat is after when she exposes her stomach to you, the second you approach her with your hand you can expect the claws to come out.