Lymph nodes are small oval organs of the immune system. They are found all over the body and are connected by small ducts called the lymphatic channels. Together, lymph nodes and lymphatic channels comprise the lymphoid system. Certain areas of the body have increased number of lymph nodes. These include armpits, groin areas, and the neck.
A small bump in your armpit can certainly be a reactive lymph node. Generally. Lymph nodes should not be felt unless you have recently had an infection or an inflammatory process. There are other reasons why lymph nodes can swell, the most concerning of which would be a cancerous process that could spread to an armpit node. Most common cancer that metastasized to the armpit lymph nodes is breast cancer.
This is not to say that you have cancer. More likely, you may have had a reactive process such as a small infection that could have caused the node to swell, if this is in fact a lymph node.
Other possibilities include. A sebaceous cyst (infected hair follicle), lipoma (fat nodule), hidradenitis (infection in the sweat glands and hair follicles of the skin).
The best way to correctly diagnose the bump in your armpit would be to see your primary care doctor
who can examine the bump and direct the workup and, if necessary, further treatment.