What Causes Brown Rash On The Chest?

I think I might have a problem...I'm a male at age 14 and have had this brown rash right under my left Breast. I'm currently putting Shea butter to see if that will help. Ive had this rash since I was 4 maybe and never researched about it or consulted a dermatologist. I really want the "rash" off my chest but all I was informed about it was that I got it when I was sick and had a fever. No itching or burning has ever occured. Please help, I can not find a solution to what this "brown rash" might be