Hello and thanks for the query
The lump may be enlarged lymph node
, or a malignant type. If its size is increasing, causing pain and effecting your daily activities than you should be worried and get treatment for this.
Most likely in armpit there is a enlarged lymph nodes caused due to infection. inflammation.
A simple test FNAC can be done to find out the cause of the lump and accordingly decision can be made for removing it by surgery. Consult your surgeon for the investigation and further treatment.