Hello,
It can be due to moisture(due to sweating or frequent washing), the abrasion caused by clothing, the pressure of sitting ,chemical irritants like laundry soaps, colognes and scented toilet paper, infections like STD’s and pinworms, anal abrasions and fissures and skin problems such as psoriasis,
heat rash, sweat dermatitis, seborrhea and eczema.
Take some antihistaminic like Benadryl or Claritin which are available over the counter.