What Causes Loss Of Appetite And Mood Swings In A Child?

My Five year old daughter has had loss of appetite over the past few days and complains that all food tastes “sour”, she has also thrown up one time and had discolored almost yellowish stool that is somewhat loose and very smelly over the past few days. She has always been a picky eater but will not eat hardly anything now because nothing tastes good. Not sure what it could be I am getting concerned as she is a smaller build but I can see that she is losing weight. She has also been moody and not sleeping well