Gynecomastia is an enlargement or swelling of the breast tissue in males (boys and men) which is normally caused by an in balance of testosterone levels (hormones estrogen and testosterone). It is not unusual that this enlargement occurs on only one or both male breast. Sometimes this swelling can cause some gynecomastia pain on one (gynecomastia one side) or gynecomastia pain both breasts which will need to be diagnosed and treated.