What Causes Itchy Brown Spots On Stomach And Back?

Hi I am a 26 year old female. Height is 5'2". Weight is 130 pounds. I am experiencing very itchy brown spots on the sides of my stomach and back. I noticed about a year ago I had a single spot on the top of my foot that was very itchy. That has gone away. But I guess it has come back only this time on my torso. I have tried using cortisone cream and they still itch like crazy. I have recently noticed new ones starting to appear closer to the middle of the stomach and under my breast. What is this and what should I do to treat this?