Aww, diaper rash talaga yan most likely, mama! A few of us moms had the same concern, one of my friends' baby had a red patch sa butt after nag-switch kami ng diaper brand, kaya ayun, nag-go back kami and nag-improve agad. Since nag-check up na kayo, sigurado namang hindi something serious, pero make sure to keep the area super dry, change diapers frequently, at maybe try a different brand if kailangan or use barrier cream like Bepanthen while you're at it. Hope baby feels better soon!