What Causes Appearance Of Itchy Rashes At Night In Children?

My son seems to get a rash at night when he is about to go to bed but in the morning it is fine. When it is bed time appears. What ever time that may be 8pm or if it is 10pm like today. He is 8 years old and it has never happened before. It is has been three days in a row now . He is very itchy and i am very concerned