Hello Constipation is a fairly common problem at this age coz of habits like less intake of fluids, inadequate toilet training and fear of using the toilet. However if you have to rule out organic causes is she thriving well-her weight and height -if thats normal lactose intolerance less likely and even so primary is less likely at this age. Give her -plenty of fruits and vegetables , less of biscuits and maida toilet train her for morning hours as thats when our body reflex is trying to eliminate stools. i would say observe her with above measures if she is growing well !!