Does anyone know what causes this rash?

[Miaserrano]

Hello, this spring/summer is the fourth year on the row that I get a random rash. The first time that I got it was on my fore arm and the more I would itch it the more it would spread. I went to the doctor and she had no clue what is was, her guess was that I had an allergic reaction to something, so she prescribed me cortizone cream. It didn't work:( it eventually disappeared. Every year it appears on different parts of my body                 ( stomach, under my breast, upper rib cage and now lower back) except that it's been two months and it hasn't gone away. I've tried different allergy over the counter meds and creams, and nothing seems to be helping.
Any suggestions on what I should try next?