could it be a rash or an irration to something. I know I get fine bumps from the sun, or from certain detergents..you might be allergic to a new lotion, colon, powder, a new fabric.
Think about anything you tried different, sometimes not all you body skin reacts to same and that area may be irriated by something.
Also try some parasite cleanses. maybe something is ventured into your body ( i know that sounds scary ) but try a cleanse.