Hi,
Welcome to ehealthforum,
It is difficult to comment on the cause for the lump under arm that you describe without examining you. The possibilities include breast tissue, infective or inflamed lymph node, folliculitis etc. Consider visiting your doctor/gynecologist at the earliest for thorough examination and further evaluation to look for the underlying cause. Treatment as needed (based on examination findings and investigations results) would be started to help you control the symptoms. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately. Drink plenty of water. Take adequate rest. Maintain healthy diet and lifestyle.
Take care.