The most common cause is irritant or contact dermatitis. There are other causes that are less common, medications, supplements, hormone imbalance, etc. An exam by a dermatologist is a good place to start.

If you sweat a lot, then it could be a fungal infection.

This is usually due to eczema, but can be a sign of other conditions such as Paget's disease (this is a different disease from Paget's disease of bone). Your primary healthcare provider or dermatologist will be able to differentiate the various cause of this condition and treat its cause appropriately.

Sometimes it is contact dermatitis or a simple yeast infection. I suggest you see your ob/gyn and let them check it out.