There are several common causes of lumps under the armpit.
If it is in the skin, the most common cause would be an obstructed sweat gland or a skin cyst. Sometimes these can get infected, and that would typically cause redness and tenderness, and the tenderness is typically more than just a little tenderness.
A lump that is deeper in the armpit would be a swollen lymph node. Sudden swelling of a lymph node in the armpit typically occurs as a reaction to germ entering the body in this region of the body, including the breast and the upper extremity. Germs can enter the body from infection, but also from any break in the skin, such as cuts or scrapes of the skin.
Since it appears to be in the skin and is only slightly tender, there are some home interventions that can be done to help promote healing. The primary intervention would be the application of hot compresses to the area, three times per day, for 15-20 minutes each time. The area should be cleaned with soap and water daily. And you can apply an over the counter topical antibiotic, such as Neosporin.
With these interventions, it will typically heal over 2-3 days. If it persists or if significant tenderness and redness develops, then there is more concern that it is getting infected and oral antibiotics may be needed, so it would be appropriate to contact your doctor at that point.
If I can provide any additional information, please let me know.