What Causes Chronic Nausea Post A Bout Of Gastroenteritis?

My 7 year old Daughter had a stomach flu early January this year after returning to school after xmas break, the flu passed after 10 days but she has constant nausea since then. She hasn t vomited from the nausea but it s starting to affect her daily at school and at home and activities are being missed. She had a blood test & urine test for IBS and other bowel problems, everything checked back normal, also she had an x-ray and the paediatrician said she was looking constipated and suggested Restoralax. The restoralax gave her worse stomach pain. She normally has a bowel movement every second day, this has been normal for her since starting solid foods. Not sure what to check into for the nausea next?