From the picture you have posted, it looks like a type of eczema ( but we need to confirm the diagnosis with proper history and also examine other areas as well). Once we have confirmed the diagnosis you can use steroid based cream and this type of allergy usually subsides completely within few weeks. You may need to change few creams as it improves otherwise this type of allergy can come back.
Next Steps
Consult a qualified dermatologist for proper prescription. you may need to take some antihistaminic tablets as per your body weight.
Health Tips
Avoid scratching or rubbing if possible as it can get worse if you do that.Avoid dettol or harsh soaps on affected area.