Sounds normal. Babies and toddlers just think belly=baby because mommies carry babies in their bellies. They can't conceptualize a baby INSIDE the belly because they can't see it. So the belly IS the baby. :) If she is breast fed or has seen breastfeeding then it sounds like shes's trying to "nurse" the "baby" :) sounds really sweet. She's acting out the things she's seeing momma do. :)