My 19m old son gets bumpy pimple like skin on his bottom. It's just on the butt cheeks and it comes and goes. It's not red or irritated by the normal rash area. The part with the bumps seems itchy cause he will occasionally try to go to town scratching it. Does that sound like heat rash from the diapers containing sweat? or could it be a yeast rash that is just not that irritated?