My baby has a red raised rash and dry skin on his back but no where else. we haven't changed detergents or bath wash. what could it be?

What did he lay on ?: If this is localized to an area so well defined, you would want to look into what surfaces he laid on within the preceding 12-18 hrs before the rash.A baby will sweat some & may dissolve fabric softener, cleaning products or other agents were left in any concentration on a surface.Some babies will react to a wool blanket that way, etc.I can think of no illness or food item that would produce this.