Work out itch
Salt in sweat can be irritating to the skin, & some sweat ducts & hair follicles may be blocked. Occurs especially in hot, humid environments. Wash areas with mild soap, rinse with cool water, pat dry. Use air conditioning or go to areas with it. Much more info at http://www.Emedicinehealth.Com/heat_rash
if not improved, see fp to check for other causes.