My three-year old daughter is vomiting yellow fluid but has no other symptoms. she's her normal self aside from the vomiting.

Gastroenteritis: If she is vomiting yellow but acting normally, no fever, irritability,or diarrhea she is most likely suffering from gastroenteritis. This is usually viral and self limited, ending in the vomiting going away and some diarrhea. Try to keep her hydrated with small, frequent amounts of gatorade or other electrolyte drinks. If she becomes very sleepy or can keep NOTHING down, she needs to see someone.