It happens to me for two reasons. One is intolerances/allergies, for me particularly eggs, dairy gluten and nuts, the other is wearing something with elastic around my waist. Never been able to to that, my stomach keeps swelling as the elastic gets tighter and tighter, so weird. I have had 3 kids but this happened even as a child.
Started Primal June 2012 at 148.5lbs, and 5' 1", reached goal weight in 5 months.
Lowest weight 93lbs - too thin. Now stable at around 100lbs much better weight for me at my age.
Primal, minus eggs, dairy and a myriad of other allergens.