rash on toddlers butt..

Posting from my phone.

Ok so the past few weeks my toddler has been scratching both butt cheeks in approximately the same place like a mirror. Well this weekend a red rash appeared on one side and is starting to develop on the other. He gets a bath twice a week or so and i started putting onlotion and diaper rash cream but it hasnt helped. We havent changed anything laundry detergent wise. We use a plastic cushion potty seat for him and have been since september. He wears the same diapers as aleays when we go out or at night and commando at home because he pees right away with underwear on.

Cant post pics but there is the link if it works.

http://m1177.photobucket.com/albumview/albums/rydersmommygraphics/IMG_20130107_200910.jpg.html?o=0&newest=1