Hi and thanks for using HCM. It may have been the soap - but that should have caused a general rash unless you scrubbed the gluteal area particularly hard with the new soap. Since it is only the buttocks that are afflicted, I am wondering if you may have sat on or brushed against an irritant that has caused an allergic dermatitis. This would explain it appearing and disappearing until you go to bed. Do you sleep on your back? Whatever the origin of irritation is, the treatment is still applicable. You could get over-the-counter antihistamines such as loratadine which is non-sedating during the day and chlorpheniramine for night so that you are able to sleep better. Applying a cooling lotion such as calamine (also available over-the-counter) for a short period (3 to 5 days) will soothe the area. Storing the lotion in the fridge will really help. This simple treatment should ease your symptoms. Once better, moisturize the area with a simple skin lotion as dry skin encourages itching. Check your night clothes and linen to make sure there is nothing on its surface that might be irritating you. Hope this helps. Thanks Dr Shabnam