Thanks for the query.
The red rash
and lump in arm pit may not be related.
The lumps can be a cyst or a bacterial furuncle
Treatment with antibiotics might be required.
A photograph would have helped in giving a closer diagnosis
and to decide whether biopsy
needed or not.
Please meet a dermatologist
and discuss regarding this.
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.