it could be caused by virus, especially this time of the year. It could be what is called Rosaola, a virus that causes fever on and off for about 3 days, then rash all over, fever stops. But from what I know, Rosaola usually happens to babies between 6 months to 2 years old. Your baby might have other type of virus. Not much you can do, just dress the baby loosely. If you are breastfeeding, try to feed the baby as often as possible to ensure him/her getting enough water.