Thanks for writing here.
It seems you are having intertrigo
It occurs due to constant friction of skin.
Use loose undergarments.
Go for weight reduction if you have extra weight.
Keep area dry and clean.
Take specail care during your menstural peroid since secretions and discharge can irritate the skin.
Use a combination of steroid
and antifungal cream at night.Candid
b is one such brand readily available.Apply it for few days only.
At day time you need to apply a good antifungal powder .
Remeber it is a common problem affecting females.
Hope this will help you.Take care.