Let someone know. If the only adverse reaction is a fever then the reaction is unlikely to be dangerous/significant - patients recieving transfusions are sometimes pre-treated with tylenol (acetaminophen) and Benadryl to prevent these minor reactions from being too symptomatic. However, a high fever should be noted by the provider giving the blood transfusion and they may send the unit back to the blood bank for re-evaluation.
Tell the nurse. Tell the nurse, who should stop the transfusion and tell the physian. Your physician will want to do a work up to see if it really was a transfusion reaction.