Welcome to HCM,
As your mammogram
is turned normal, nothing to worry.
as your doctor told there might be some breast soft tissue injury
with hitting some thing.
due to soft tissue injury there might be some infection in the breast giving rise to enlarge, tender lymph
nodes in armpit.
Go for one proper antibiotic medicine course after consulting your doctor and you will be alright.
After infection subsides, lymph nodes in under armpit will remain for few days.
Ok and bye.