The lump may be a lymphnode swelling or a breast part swelling .
A part of breast is also seen in the axilla(under arm) region normally, in case of breast feeding the breast are enlarged and along with this the under arm may also but it should be bilateral.
If it is unilateral you need to rule out the chance of mastitis which is inflammation of breast.
I advice you to consult a doctor so that it can be ruled out
Health Tips
redness or pain is the first site when you need a doctor