What Causes Rashes On Butt In A Kid?

My son went to sleep lastnight with no clothes on as I seem he was sleeping I was about to put his pjs on and noticed on his butt that he had a rash... It was blotchy and a little warm...when he woke up this morring it was gone! I have not changed soap, detergent,his diet or anything of that sourt...I took him to his pediatrician last week because he has had bumps that I think was eczema and she told me it was because of the weather and to moisturize 3 times a day but with my child only being 3 years old and had had it for about 2 months and not went away now with this rash I called a dermatologist and they are closed because of the new year...can you help as to what this rash is?? Thank you