I had a skin rash last week, I ending up seeing the nurse at the doctors and she got the doctor to see me. He seemed to think that chances are it was an allergy, it could be something that you have eaten before and the brand have changed a chemical or colour etc in it. He told me to use a 'soap substitute' to wash with, (either aqueous cream or the E45 washes that chemists, boots etc sell.) It still didn't go down, so they gave me some hydrocortisone cream to use on it, and that seems to be helping.
If you can't get to see your doctor yet prehaps just trying using E45 or something on it and see how it goes.