There isn't sufficient information to give you a specific diagnosis as the lesion would have to be seen to get a cleare picture. I would advise you to meet a dermatologist so that you can be examined and undergo the necessary investigations such as CBC, IgE, etc to find the underlying cause of the problem and have it treated accordingly. You can use a lactocalamine lotion over the lesion which may give you some relief and you may require an antihistamine.