My 3 year old is newly potty trained as of 2 months ago, it's been great & so nice not having to buy…

My 3 year old is newly potty trained as of 2 months ago, it's been great & so nice not having to buy or change diapers! However ever since we dropped diapers he's gotten this really bad rash all over his butt😩 Iv read that it could be his skin adjusting no longer being in a moist diaper but just wondering if anyone else has experienced this with their little ones? How long did it last?