Lump of flesh under armpit

Hi, welcome to the ehealth forum and I am glad to help you.
You seem concerned by the probable lymph node in the right axillary region and increase in size and tenderness before the menstrual period.
Lymph nodes can sometimes appear swollen or become noticeable just before the menses because of fluid retention but such symptoms are otherwise unrelated to the menses. Under the arms, there may be the same lymph tissue or there can be breast tissue fairly high going into the armpit (axilla). There is no way to diagnose this specifically other than by biopsy. However, that is not usually done unless the process is very local and not general.
You might consult your doctor to look for causes of the lymph node swelling that may be due to infection or inflammation.
Hope this helps. Take care.
Note: This post is not to emphasise final diagnosis as the same cannot be made online and is aimed just to provide medical information and no treatment suggested above be taken without face to face consultation with health care professional.