Hello~when it comes to lumps, I always think it is a good idea to have it checked by your doctor, this will give you peace of mind and you won't be "guessing" as to what it could be.
It sounds like you might have a lymph node in the axillary region (armpit area). This is often reactive (e.g., secondary to infection and/or inflammation), but can also be pathologic (e.g., metastasis secondary to breast, lymphoma, or other cancer). If it does not improve/resolve, you should see your doctor who may order imaging such as targeted ultrasound.