Sounds like it may be a fungal infection.
Make an appointment to see your doctor so that he can examine the area and prescribe the appropriate medication for you.
In the mean time do NOT use any soap, bath or shower gel as this dries the skin. Use plain water and dry gently.
You can purchase emollients from the pharmacy that can be used in stead of soap.
Use a wet tissue paper to wipe yourself clean after a bowel movement.
The doctor is likely to prescribe an appropriate medication for you, but in the meantime you can apply antiseptic creams like Sudocrem to help protect the skin and help it to heal.
Wash and dry gently and just lightly rub on the cream. Don't cover with any bandage. Wear cotton underpants. These help the skin to breathe. Nylon and manmade fabrics can cause a lot of sweating and this encourages yeast growth.