Welcome to HCM
First of all I would like to inform you that commonest single nodular swelling in groin area is enlarged Lymphatic gland(lymph node).And commonest causes of its enlargement is local infection near the area and the respective whole legs(i.e-in case of any left leg ulcer,furuncle
etc. left side of thigh lymph node will be enlarged).
But here the others possibilities is Inguinal hernia
The local examination by a doctor will easily diagnosis the case and treat accordingly.So stay with your doctor.