I have a lump in my armpits after 3-4 days of soreness. What could it be?

It is likely that you have a swollen lymph node or an infection in the lymph nodes. This often occurs as a result of viral and bacterial infections, such as the common cold or flu. An abscess can also cause swelling and soreness in the area. If your lump persists and is accompanied by other symptoms like fatigue, general malaise, fever and chills, then it may be best to get it checked out by a physician to determine its cause.

In addition to infections that could be causing this swelling, there are some other possible causes including:
Hyperthyroidism: Swollen armpit glands can occur with an overactive thyroid gland (hyperthyroidism). If left untreated, this condition can lead to serious complications like heart palpitations, fatigue and more.
Skin abscesses: Armpits often contain sweat glands which become blocked from bacteria and debris over time leading to formation of abscesses which may cause inflammation or swelling in the area.
Lymphangitis: This is an infection caused by streptococcal bacteria within lymphatic vessels near your armpit usually due to injury in the surrounding skin tissue such as a cut or insect bite near there. Symptoms include red streaks around the affected area along with pain when touched and swollen tender lymph nodes near your arm pit.
It's important to note that these potential causes can all be treated so if you think you may have any of them it's important to get medical attention right away so you don't risk further health complications down the road due to lack of treatment!