Thank you for your query
Consult a pediatric surgeon for this problem. At times it is a clinical diagnosis and the pediatric surgeon will be able to tell why the swelling is there. If it was an abscess
that is collection of pus then it would respond to antibiotics which were given to you. But if its still there then its better to get that drained. However at 5 days age an abscess is unlikely except if some intra muscular injections were given on the thigh. Second cause can be a lipoma
or collection of fat. It may not require any urgent treatment but a diagnosis is must. Some other soft tissue
tumors can also present at birth. It can also be a vascular malformation
under the skin that means abnormal growth of blood vessels. Mots of these conditions are benign and you need not worry but it is good to get diagnosis.
Hope that helps
God bless you