Some people find soap can be harsh on their skin, causing a red, itchy rash. Even more so for people who already have dry skin, as the harsh detergents in soap can strip away natural oils, leaving the skin vulnerable to irritation.
Also, some soaps contain irritants like fragrance or coloring agents that can aggravate the skin even more. So, choosing a mild soap for sensitive skin is very important. Also, consider using a moisturizing body wash to protect your skin from drying out.