My little niece have been vomiting 5days after appendectomy what could caused that?

Ileus/obstruction: Your niece's parents shouldn't wait one more minute to take her to the ER. I hope it's something minor like an ileus (sluggish G.I. tract) complicated by resuming a normal diet too soon. However, she may be experiencing a serious complication from the appendectomy like a mechanical obstruction. Vomiting can occur from appendicitis but it should NOT occur after an appendectomy.