Hello,
One possibility for your symptoms is chafing or friction rub. It can also be due to allergic reaction or contact dermatitis to your undergarments, sanitary napkins or tampons, soaps etc or fungal infections. Wash the areas several times with fresh water. Do not use any cosmetic products at the sites. You can apply some calamine lotion at the rash as it will help in soothing the skin.Also wear cotton undergarments.Application of mild corticosteroid cream and oral antiinflammatories like ibuprofen/acetaminophen are also helpful.If the symptoms are severe then it will be best to consult a dermatologist.
I hope it helps. Take care and regards.