Thanks for the query,
Lump in arm pit area can be an abscess
due to bacterial infection.
However this area is rich in vessels and hence chances of thrombophlebitis
is also there.
You may require treatment with antibiotics.
Please meet a dermatologist
for exact diagnosis and treatment as it is a very vital area.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.