It can be some lymph node swelling which should not be ignored specially if mobile and non painfully.
If it is tender to tough along with fever , then need to be drained and antibiotics therapy .
As far as nodule in left underarm is concern , few this can't not be ignored like ,
Next Steps
Do u have family history of breast Ca?
Do u have done self breast exam ?
Do u have found any nodule , lump ?
Is there any change in size and texture of nipple/ areola ? Or change in skin texture.
It can be early signs , including any watery discharge from nipples and should not ne ignored.
Tenderness can be due to underlying abscess/pus or infection.do u have fever or any other complaints or on any other harmonal medications .consult me here on practo