Is she scratching the area? Does it seem irritated in any way? If so, I'd have her checked by a vet.

Was it all of a sudden, or a gradual loss? When you pet her did hair fall off into your hands?

Does her breed normally have hair on their tummies when they're an adult?

How does the rest of the fur on her body look?

It could be this: puppies are like us in a way, we lose our baby hair and it's replaced by our "grown up" hair. Kinda like losing our baby teeth and getting our adult teeth.