Bloating , headach , nausea after meal.

My daughter in 9 year old. She started menstruation one month back.2 months prior to start of menstruation there had been episodes of bloating of stomach , headach , nausea every time she ate. These episodes were persistent. During the days of her period these problems subsided but they returned once mensturation got over. On paediatric consultation we have done whole abdomen USG,erect lateral X ray of abdomen , LFT, urine routine and culture , Stool test for three consecutive days but all reports were normal. She is currently put on laxopeg twice a day , lanzol junior one tab a day , protectis one tab a day by a paediatric gastroenterologist .She had her second period 4 days ago.It lasted for 7 days.During this period bloating subsided but there was pain in pelvic region.Hot water bag compress helped.But now again there is bloating, abdominal pain , headach and nausea. We are clueless which specialist we should consult ?Could it be cancer or any other complicated disease?