Today i saw a patient with jaundice she developed it only one day after blood transfusion so what are the causes ? could it be haemolysis? ?

Probably: Most likely this is indeed hemolysis. It's usually but not always possible to pick up sensitivity to donor blood by a recipient; the Kidd blood group antibodies are expecially treacherous here. There are a variety of other causes of jaundice, including liver disease and bile duct obstruction. Blood transfusion saves many lives but the risks must always be weighed against benefits.