You ae most probably suffering from Taenia infection- a fungal Infection of the undergarments area between the legs and Groin-the area between the thighs and abdomen. Most commonly it is seen in moist areas usually made so by sweat. The antecedent cause in most cases is getting underclothes laundered by usually a launderer and using dirty water to do so. This also underscores the point that one s underclothes should be washed by oneself always. The treatment ,the best one is Ointment Parag Matar if you belong to India. If you are an enquirer from outside India it will be difficult to procure the above mentioned oint. So use Cotrimazole oint or Nizral Oint. The latter has Ketoconazole as active ingredient is quite effective. Hope this helps.