A hard, pea-sized lump in the armpit is usually a swollen lymph node. The lump can appear especially when a person is already experiencing an illness or symptoms of an infection. The glands under the armpit tend to swell due to the presence of a health condition in the body, as stated by NHS.
There is no cause of concern if a person has such lumps under the arm as they tend to disappear when the underlying condition is treated. Some people tend to think that the lump in the armpit is a lymphoma, cancer of the lymph glands. This may be not be correct, but checking with a doctor can clear any doubts. The lump can also be a skin tag, which is a harmless lump that hangs off the skin. Skin tags may be left alone because they don't cause any harm.
Lumps are usually common and can be caused by an injury or an infection. Most lumps are not painful, but if it causes unexplained pain, it is necessary to seek a medical diagnosis. The doctor will determine if the lump is a symptom of a disease or caused by an underlying condition, as stated by NHS.