I am a very fit/active 46 year old female (exercise 6 days a week - mostly high intensity exercise).  I often have a small lump under my left armpit just prior to my period. I have not had a period now for 6 weeks ( I have been irregular with my periods for probably 6-12 months).  The lump is getting larger and is visible if I wear a singlet (it is also palpable).  I also have regular night and day sweats.  Just wondering what I should do  - or If I should see a doctor about these symptoms.
You should see a doctor. A complete clinical exam including clinical breast examination, mammogram and fine needle aspiration for cytology of the lump should be done. Please discuss it with your doctor.