Hi, thanks for your query.
As per your description of the rash and its areas of involvement it seems to be a fungal rash
It occurs in the areas which are prone to be moist like body folds. moisture and sweat facilitates the growth of fungi.
To treat the same you may require a course of oral anti fungal drugs along with local anti fungal creams, and oral anti histamines
You may consult a dermatologist
for exact diagnosis and appropriate treatment.
Hope this solves your query.
You may write back if you have further queries in this regard.
Wish you a good health.