What Causes Rashes On Body In Infant After Recovering From Fever?

Hi my 1 year old son has not been himself for about 10 days now. He had a fever of 102-103 for 3 days, diarrhea, drowsiness( sleeping more, always tired), and now the fever has subsided but I saw a rash on his body (stomach, back, behind ears and a little on his face). What could this be, could it be serious, or just a common virus?