Well heres my gerbil story. i have 3 gerbils and one noticably started getting bigger. not neessarily in the tummy but never the less, big. then she built a nest. so i took the other 2 to the petsmart and found out that one of my 3 supposedly "girl" gerbils is a male. its been bout 2 weeks since the nest but she doesnt look pear shaped. could she just not be "ballooning out"?