Explaining Lupus to a child

My son is 4 years old and I have SLE.  While we were outside playing today, I told him that I needed to go sit in the shade.  He asked why I was sitting in the shade and not playing with him.  I asked him to come over and play with me in the shade, but he said he was busy playing in Zach's old garden.  He became very angry with me that I would not play with him.  How do I explain to him that I am sick and can not be out in the sun?  It was WAY to hot and humid to put on the large brimmed hat and long sleeves.  I do not know how to explain this to him without frightening him.  He has already experienced the loss of his twin brother and do not want him worrying that I will die too.  At this time my Lupus is under control and I am doing well.  He does not remember the time that I was having a horrible flare.  Any advice would be greatly appreciated.

Krista, Mom to HLHS Angel Zachary (5/27/05-10/17/06) and his twin brother, Dylan.