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Looks like you have developed a lesion of Vector dermatitis, also known as Paederus dermatitis
Paederus dermatitis is a peculiar, irritant contact dermatitis
characterized by a sudden onset of red lesions on exposed areas of the body.
The disease is provoked by an insect belonging to the genus Paederus.
Now as in your case the lesion is persisting since 4 days, you need to apply an antibiotic/steroidcombination cream over the lesion twice a day.
Take oral antihistamine like levocetrizine once a day to decrease the swelling .
Do not use any moisturizer over the lesion till the time the lesion heals
Hope it helps
Wishing you speedy recovery
Dr Geetika Paul