There are many possible diagnoses for the “lump under your armpit” ; you chose the worst one. A breast cancer can metastasize in the armpit’s lymph nodes, which can manifest as a lump under the armpit. But it is a very rare possibility in case of 16 years old girl. Other conditions conditions can cause “lump under the armpit” such as a: furuncle (ingrown hair), inflamed lymph node, fat gland cyst, skin tumor etc. You can consult your GP for a precise diagnosis.