Hi,
Well an armpit lump is usually benign but is investigated till a confirmed diagnosis is available and malignancy is ruled out.
In your case, the size of the lump is large and therefore a biopsy must have been done.
Also atypical cells would need to be investigated further to rule out any associated breast lesion.
What is your age? Have you noticed any abnormal features on breast self exam?
Ask your doctor for a copy of the biopsy report.
Get the MRI done and wait for the results to come in.
What is your doctor keeping as the most probable diagnosis in this case?
Do let us know about your progress and also do post in the biopsy report.
Hope this helps.
Good luck.