Need to see.. Probably fungus...Fungal infection.. Also called jock itch, when occurs in groin.. Usually fungal infection is common in moist areas of body like skin folds..It's mostly circular or round in appearance.. May be red or black in colour.. Treatment depending on the type and severity.. Avoid sharing clothes, towels and soap; sweating also increases it..As it has to be differentiated from eczema, bacterial and viral infection, doctor need to see to confirm the diagnosis..So, meet me in my clinic or do direct online consultation by sending photos for detailed information and medicine for treatment.