Ay, rashes sa leeg can be tricky kasi pwedeng maraming causes, heat, sweat, irritation from clothing, or sometimes allergy, ganyan. I'd really suggest checking with your pedia or dermatologist para malaman kung ano talaga ang dahilan, kasi ang treatment ay depende sa root cause niya. One of my mum friends had the same issue sa daughter niya and it turned out to be yeast infection, kaya kailangan ng specific cream prescribed ng doctor. In the meantime, keep the area clean and dry, and try cotton clothing na hindi makakalusot masyado. Hope your baby feels better soon, mama!