Hi M.,
I experienced it as a child and I've heard it before and usually there is a nutritional issue as she may be retaining water or there is gas. Her digestion is just not quite right. Is she constipated regularly or is there a tendency to diarrhea? I had a long term digestive illness. The doctors diagnosed me correctly but treated me incorrectly and caused years of problems.
Healthy is a relative term these days. I would need to know a little more about her eating habits and her environment. It's nothing that should scare you though you should pay enough attention to it to fix it.
If you want to give me more information I'll try to help or you could go to a local health food store or a naturalist to diagnose. Most doctors would say nothing about it.
Regards,
M.