Have you touched anything that you do not normally touch?
Sounds like a reaction to some allergen. After washing the area, apply an antibacterial cream to it for a few days. Or an antihistamine.
Can you put the pic on your profile?
My first guess would be a soap or chemical or plant irritation. The arm was the site of my Celiac-related skin breakouts, but it was a slow progression of spots, not all at once.