Brief Answer:
I would like to get her evaluated to be on safer side

Detailed Answer:
Hello

Thanks for the query

The slight pulsatile movement of abdomen when holding breath is common particularly in thin built person and when abdomen is scaphoid.This is due to the pulsatile movement of the abdominal vessel(aorta). As presently she is suffering from stomach flu the pain may be due to intestinal spasm which usually resolves over a period of time after taking some antispasmodic.The pain in stomach flu is crampy in nature not pulsatile.
If pulsation is such that it can be seen clearly while breathing and pain is pulsatile/throbbing then I would like to get her clinically examined by a physician(USG abdomen screening may be needed) to rule out aortic aneurysm if any(this condition is very less common).The aortic aneurysm may remain undetected and suddenly get symptomatic.It can cause pain due to compression of the surrounding structure or stretching of the wall.
Keep her hydrated well by giving plenty of fluids.
I shall be glad to answer if you have any more doubts.
Wish you and your child good health.
Regards

---

Brief Answer:
Antispasmodic like dicyclomine or drotaverine

Detailed Answer:
Thanks for the follow up

You can give antispasmodics like dicyclomine, drotaverine(drotin tablet) or hyoscine butyl bromide(buscopan tablet), two to three times daily depending on the severity or frequency.All these are prescription medicine and please get it prescribed by a physician.
Regards

---

Brief Answer:
If the pain is severe and throbbing in nature get her to ER

Detailed Answer:
If the pulsation is visible and pain is severe,throbbing I would like to get her evaluated immediately.

---