Q: There's a painful lump in my armpit. What could it be?

Does it look red and inflamed or filled with pus? If so, it may just be an ingrown hair or infected follicle from shaving or using antiperspirant. Avoid shaving and applying product there, clean the area gently with soap in the shower, and apply warm compresses several times a day for a few days, and it should clear up.

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Another possibility: You have a lipoma, which is a knot of fatty tissue that commonly grows in places like the shoulders, neck, and armpits. While you can get them at any age, they mostly form in adults between 40 and 60. They're almost always harmless and painless. However, one may cause pain if it lies on any nerves. If it bothers you, your doctor can remove it, typically by making a small incision and taking out the tissue.

Or you could have a swollen lymph node. Predominantly located in the neck, groin, and underarm areas, lymph nodes act as filters to trap "intruders" in your body (think germs and cancer cells). They can become painful and enlarged when you have an infection, like strep throat or mononucleosis. The swelling and discomfort usually go away when the infection does. Some women also have small amounts of breast tissue near the armpit, so if you notice soreness just before your period, it may be due to the same hormonal changes that cause period-related breast tenderness.

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If the lump doesn't disappear in a couple of weeks or gets bigger, or if the pain seems to worsen, it could be a cyst, a breast infection or (very rarely) a tumor. See your doctor to get it checked out.

Health’s medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine.