Hi,
This is due to the swelling of a lymph node, which reacts to an infection in the nearby area and subsides of its own after the infection is over; but while going back, they may not shrink completely. Arm pit lymph node swelling is also very common due to increased usage of of antiperspirants, in such cases, allow skin sweating and wash the area clean instead of preventing perspiration.
Solace
Its not something serious. But if it persists I would suggest a visit to your family doctor for starters
Take care