Lump under your armpit may be due to enlarged lymphnode or Lipoma
.Most of the times,lipoma is painless.
So,pain may be due to infected and inflammed lymphnode.
to any site will detect only bony diseases,not soft tissue
Consult your General surgeon
and get examined clinically and investigated.
Simple investigation like Fine needle aspiration cytology(FNAC) will clinch the diagnosis.
Proceed as per your doctors advice.
Wish you good health