I'm not a doctor and definitely no expert, but this sounds like a form of breast cancer called Lobular breast cancer. This is a cancer which starts in the lobules, which are the milk producing glands. 

You can find information here https://www.breastcancercare.org.uk/information-support/facing-breast-cancer/diagnosed-breast-cancer/primary-breast-cancer/invasive-lobular-breast-cancer 

and here http://www.cancerresearchuk.org/about-cancer/breast-cancer/stages-types-grades/types/invasive-lobular-breast-cancer

You may need a few more tests and scans, but once they're done your oncologist will be able to prepare a treatment plan. 

I'm sure this is a dreadful shock to you but breast cancer treatments and outcomes have advanced significantly in recent years. For now, fingers crossed that it's been caught early.