Hello,
This armpit swelling can be due to a boil, cyst or a swollen lymph node. Lymphadenopathy (swollen lymph nodes) is usually caused by a viral or bacterial infection along with other numerous causes, malignancy being one of them. Sebaceous cyst is a closed sac or cyst that fills with a fatty white, semi-solid material called sebum produced by sebaceous glands of the epidermis. Boils are caused by infection of the hair follicle be staph aureus. Treatment includes antibiotics.
I suggest you to get an evaluation done from your doctor and get a FNAC ( fine needle aspiration cytology)of the lump done from him. This will confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Typically it's an indication that your body is fighting infection, swollen lymph nodes. Most likely this would also occur in neck and inner thighs as well, but you do not report this. Breast cancer is an obvious worry due to the location and size. However, it could be something simply related to your period. Breast cysts are very common in women and tend to inflame around the time of your period; though one would expect these to occur symmetrically in both breasts. It's a mild and common disorder that usually goes undiagnosed. Essentially, the blood and fluids get trapped in the breast area around menstruation and take longer than usual to be released. An ultra sound should have caught this. I'm afraid I don't have enough info to guide you in the right direction.
the armpit swelling might be a lymph node, though its weird the ultrasound did not catch it
anyway u need to find the reason
it can be not even connected to your breast
if ur nodes are swollen, you can have problems with any femininne organs
my mom;s lymph nodes under arm pits were swollen for a year prior to discovering she had hysteromyoma
get a full check
Not sure how to edit my question but I wanted to add that my period has been irregular for the last 6 months or so. I only get it every couple months. It is normal when I do get it though.