Aww mama, first time mom stress talaga yan! Ang itching sa kwatan usually it's either sweat buildup, heat rash, or maybe friction from diaper/clothing, try keeping the area super dry, change to breathable cotton clothes, at consider switching to a diaper brand na gentler if you haven't yet. Pero since baguhan ka pa at medyo worried, mas safe na consult with your pedia para ma-rule out if may skin condition na kailangan ng specific treatment, especially pag tumatagal pa or lalo pang tumataas ang kati.