Ay, naiintindihan ko ang stress mo, mama! Kuntil near the butt area could be a few things, diaper rash, heat rash, or sometimes just friction from diapers or clothing, lalo na kung mainit ang panahon, pero ang slight swelling na kasama nito, gusto ko talaga na ipatingin mo ng mabilis sa pediatrician para sure tayo kung ano exactly at ma-treat agad nang proper. One of my mum friends nag-panic din dito, at nabilis mawala ng concern once ma-check ng doctor, so don't worry too much but best kunin talaga ang peace of mind mo right away, mama.