The development of any mass or new growth should always be evaluated by a qualified physician. Since we cannot be sure what this is from your description, it is probably best for you to see a primary care physician
such as a family doctor
or internal medicine doctor
Your lump could be one of several different things. Sometimes when we get an infection, lymph nodes in the area can enlarge and be felt. There are lymph nodes under your arm pit. Sometimes small benign tumors can form. One such tumor is a lipoma, which is just a small mass of fatty tissue that has enlarged. Sometimes a previous infection or minor trauma to an area can form a ball of scar tissue of a cyst which can feel like a hard painless lump. In very rare cases, these masses can be a malignant tumor of the soft tissue called a sarcoma. Because of these rare, but dangerous possibilities, you should be evaluated soon.
I suggest that you schedule an appointment to see your primary care physician. He or she can take a close look at this lump and determine what it most likely is. If it is benign, but still bothersome to you, then you could have it removed surgically. Good luck.