More than likely, irritation from the soap. Your best bet is to use a very mild soap (or non-allergenic), or just water, to wash under the foreskin, and always rinse well.
My mother-in-law was having cancer treatments, and when she would style her hair and spray it with hair spray, instantaneously red dots like pinpricks would appear on her skin. It never had happened to her before; obviously her immune system was compromised and that made her skin really sensitive either to the product or its perfume. If anything about your health suggests a compromised immune system (such as, for example, diabetes or being on a medication), be especially careful with soaps, lotions, and clothes detergents. Use only those without scent or that are marketed as being hypoallergenic.