It is hard to tell what the lump is without seeing it. It might be an inflammation of the skin, of certain of the skin glands (sweat or sebaceous glands), or even the breast gland tissue. The lower side of the breast is exposed to heat and moisture due to sweating. These conditions might cause inflammation, which might even be accompanied with infection. However, with this inflammation usually there is redness, too.
As told before the lump might be due to mastitis which is an inflammation of the breast tissue. The inflamed tissue is painful and swollen. There might be redness present if the tissue is close enough to the skin. If the tissue is close to the nipple, and infection is present, there might be fluid draining from the nipple.
In the end, it is possible that the lump might be a tumor. Whether it might be benign or malignant cannot be determined for sure.
As you can see, there are many possible diagnoses for the breast lump which vary from dangerous to those with the smallest risk of further complications and consequences. Therefore, your sister is advised to visit her family doctor. He would make the basic examination and might be able to recognize what specialist should diagnose the lump and further refer her.
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