Aiyoh, red rash on neck bisa jadi banyak things lah, bisa dari heat, friction dari baju, or sometimes allergic reaction to laundry detergent or skincare. A few of us mums noticed our kids get these when weather panas or if they're wearing tight collars, especially during this time of year. Since it's your first time, I'd suggest checking with your doctor or paediatrician to confirm what it is, better safe than sorry, kan, so you can treat it properly without guessing-guessing.
awak takut ni allergic reaction juga la. kalau makin teruk pergi check doktor please