I experienced this recently with my DS and we went to the emergency room when his temp got over 39 (I measured 39.2 and it was 39.8 when taken at the hossy) It turned out he had a UTI and we were admitted for IV antibiotics for 36hrs till they identified the bug and could have orals. He had no other symptoms but the fever but I'm gald I took him in. As the fever is increasing I would go to the doc or the ED.