Going for motion after feed is nearly normal. It is known as gastrocolic reflex, which need not be given seriousness. The way you are thinking that, since she passes motion immediately after food and not gaining weight is a misconception. Motion comes after food crosses ~6 metres of intestine. So the motion which comes out is previous day 's food.
Next Steps
21kg at 8 years is not a bad weight. If she is active in studying and playing, then you need not worry. In 2or 3 more years she ll gain wieght due to pubertal growth spurt