Why does this happen to me?

Hi im 15 years old and ever since i was a baby every year around april i get this horrible rash on the back ok my knees i get itchy othe places but not like a rash, its sooo itchy and red and cuts from scratching it usually ends around july and never comes during winter its soo werid cause know one else i know has this but its always in the same spot and its soo weird...i think its from allergies and i have allergy meds and creams and stuf but why is this rash always in the same spot??? Whys it there???