I Noticed A Red Horse Shoe Shaped Rash (NOT Raised...NO

I noticed a red horse shoe shaped rash (NOT raised...NO itchy...NO Pain...No bumps) with white flaky center...it started as a pea size spot just beyond my right arm pit (back side)...now it is a size of a quarter...it has grown within 24 hours...I have applied tea tree oil on it tonight (only)...any advice to give...