Firstly stop doing all that! Just wash with regular body soap and let the water fully wash it off. Secondly, make sure to change your underwear and pants daily. Thirdly, switch your laundry detergent to a “free and gentle” version. Perhaps the soap/detergent you’re using on your underwear/pants is the irritant. If you do all this, and it is still a problem, perhaps consider asking a dermatologist?