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.
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.