Pain can occur under your arm for a number of reasons 1.Certain infections can cause pain under the arm ,Lymphangitis is caused by an acute streptococcal infection of the skin that infects the lymph node ducts, and many of these ducts exist under the arm. This can cause pain Other infections, such as skin abscesses, can cause lumps to form under the arm that are painful. Hair follicles can become infected and a large lesion can develop. Sometimes these abscesses require drainage and an antibiotic. 2.Lumps under the armpit are often painful, and they have several causes. Cancer, such as Hodgkin s lymphoma and metastasized breast cancer, can cause a painful lump to form in the armpit suggestions If the lump is due to infection caused by bacteria, it should go away with a course of antibiotics in few cases, an extremely painful knot under the armpit may need to be surgically removed. Hope this is helpful