You seem to be having a cystic swelling in your arm pit. It could be due to some infection or because of frequent shaving and application of antiperspirants. You need to consult a doctor, to get it examined.As a general precautionary measure-
do not try to squeeze it as it can lead to further infection
Keep the area clean by using any antibacterial(betadine
apply antibiotic cream
The definite treatment will either be in form of oral medicines or removal of the cyst. Hope that answers your query. Take care.