It is possible that the stomach is big because your grand daughter is on a healthy diet as you say but if you think that something is odd it would be wise to seek medical attention. Call your pediatrician and explain what concerns you.
Did you notice some other symptoms like a vomiting or high fever or some mood changes?
You are probably aware that the most common condition for newborn babies is baby colic.
One of the symptoms of baby colic is has a hard, distended stomach, with knees pulled to the chest. If you have noticed this than it is better to consult with your pediatrician than to trying some home remedies and supplements.
Let us know about the diagnosis and treatment options.