Hi there.. the neck rash could be caused by moisture and heat combined. Try to keep the area dry. You can apply some powder to help with that. If it gets worse, you may need to consult a doctor to get it properly diagnosed.
Tips: 1. Wash with breastmilk and let it sit a while 2. Wipe with cool boiled water and let it dry 3. Apply moisturiser 4. Repeat steps
My doc recommended moisturising after wiping clean too