Around the arm pit
can contain breast
tissue, lymph nodes, as well as other tissues. I would strongly suggest seeing a doctor. It could be nothing more than a cyst, a fat or fluid filled lump. And if she's been ill it could possibly be a swollen lymph node. However, breast tissue can sometimes be found near the arm pit
, so it's theoretically possible that it could be related to that.
What does the lump feel like? Is it sore
at all? Does it move freely?