You may be having fungal infection under your armpits. The treatment is simple. You have to take anti fungal tablets and creams for few weeks following which it resolves. Do not apply soap on that area and do not self treat with over the counter steroid creams. Kindly send photos for accurate diagnosis or visit a dermatologist. If this is fungal infection, it needs to be treated as early as possible as it will spread further on your body  and also to other people in the household.