What Causes Stomach Ache On A Child?

Have 2and half grand child temp 38.1 says her tummy sore. Picky eater but drinking tea and water fine. Dr prescribed ponstan twice a day...after taking she is a bit lethargic but then gets second wind andis playing normal ....then after about 3 hrs temp rises again and she is weepy and sor again. Normal stools and urine working. Worriried about yhe cramps and tummy that she pulls up her legs in pain ??? Needsome advice please.