Usually they appear quite pear shaped then some with larger litters get very big and round in the belly. Most will start building massive nests out of their substrate within 24 hours of giving birth. Near the end of their pregnancy you can often catch them sleeping full out on their backs (thought rats that are overly warm and not preggie will also do this). Hope she's not preggers at such a young age. If she housed with other females, she could also just be hogging most of the food...some are like that the piggie little boogerheads.