There are many causes for an underarm lump, many of them innocuous. Shaving and antiperspirants can cause cysts and infections. Using a deodorant instead of an antiperspirant can help to prevent an infection in the future.Fatty tissue (lipoma) is another growth that can appear in the axilla area as well as boils and abscesses. Sometimes the lymph nodes can become swollen as a result of a virus, vaccination or bacterial infection.As you can see,the possibilities could be many,but the best thing to do is to have this large lump checked out by your doctor who will examine you and might also order some type of test so a correct diagnosis can be made.If it's an infection you would need medication to have this problem resolved
I hope it's nothing serious,but please have this lump checked out soon okay?