A woman may have swellings in the armpits which may be on one side or both. These swellings may be associated with pain, in which case they are inflammatory or infective swellings, or they may be painless. The various  causes of axillary swellings are as follows:

Accessory Axillary Breast Tissue

Prominent Axillary Breast Tissue: Normally, there is some amount of breast tissue which is present in the armpit. It is an extension of normal breast tissue and is called the Axillary Tail of Spence. In some women, this part may become prominent and enlarged, mimicking a lump in the axilla. They may present as bulges under the armpit. Prior to periods, it may become slighty firm and may feel heavy and slightly tender to touch. This may also  enlarge during pregnancy and lactation like normal breast tissue and may also function during pregnancy. This may be left alone if not very large but if it is unsightly for the patient and if it is bothering her, then it may be surgically excised.

Axillary lymph nodes

These present as firm to hard, solitary or multiple swellings in either one or both axillae. With proper examination and investigations, a biopsy is carried out to reach the diagnosis and accordingly further treatment is carried out.

Axillary hidradenitis

A person presents with multiple episodes of lump formation in one or both axillae. These may cause pain when they are infected and may discharge pus. The infection settles with the discharge of pus, only to recur at a later time. This is due to the infection of the multiple apocrine glands which are present in the axilla. Treatment with antibiotics may be tried, but if the infection is persistent and recurrent then excision is recommended.