Hi welcome to Health care magic forum. You are getting itching only in the night and cleared by morning. means it is triggered in the begening of the night. and cleared in the second half. It means you are coming in contact with the allergen, while sleeping. means the things you use while sleeping. Like night dress, sprays, deodorants, pillow covers, bed sheet, snacks recently started using, and you use only in the night.
Avoid using such things one by one with 3 days gap and try to find when you get relief of this rash
. from that time you avoid those things which caused the rash.
Besides i advise you some diet recommendations as, Avoid taking brinzal, fish, other sea foods, and egg which supposed to have potent allergens.
Take more of green leafy vegetables, pulses, sprouts, milk,fruits, juices and dry fruits. which increase blood, general health, and resistance to such problems. Wishing for a quick recovery and thanks for calling.