My son had the same thing two weeks ago. He had a low-degree temperature for 5 days. When the fever subsided, he broke out with a red rash all over his face. The doctor said it's called ROSEOLA virus. Here is more info about it:
WHAT IS ROSEOLA?
It is a usually harmless illness caused by a virus. It occurs almost only in children age 3 months to 3 years, most often between 9-12 months. It is probably the most common cause of fever in this age group.
WHAT DOES ROSEOLA LOOK LIKE?
This virus generally causes 3 days of high fever (often over 103). The fever then subsides, and the child breaks out in a flat or bumpy red rash, usually starting around the neck, back and chest, then spreading out. The rash lasts a few days to a couple weeks.
Roseola is about the only virus in which the rash appears after the fever breaks.
Sometimes this virus will cause 3 to 7 days of high fever, with no other symptoms and no rash. Some children will have swelling of the glands in the front and back of the neck, runny nose, cough, ear pain, vomiting or diarrhea with this illness. Children can have one or all of these symptoms.
The characteristic of Roseola is that infants don't seem very sick and act almost well when the high fever comes down.
WHEN IS IT CONTAGIOUS AND HOW IS IT TRANSMITTED?
It is contagious from about two days before the fever starts until 1 or 2 days after the fever is gone, even if the rash continues. Children who are fever-free for 1 or 2 days can return to school, even if they still have rash. It is passed via saliva, runny nose, or cough. The incubation period (time from when your child is exposed to the time of actual illness) is around 10 days.
HOW DO I TREAT ROSEOLA?
You essentially just treat any symptoms that are bothering your child. There is no actual treatment for this virus. The main thing to keep in mind is that this virus can cause high fevers. Try to be diligent in treating moderate to high fevers.
I hope it helped. Good luck !