Triple negative breast cancer

When I first had my core biopsy the results came back er+ pos- and her2-. I recently had a lumpectomy and my lymph were negative and I had clear margins. When the results came back from the lab all was different than biopsy. I was triple negative the tumor was very very small early stage 1 2.1mm and grade 3. I went to see my oncologist he told me it doesn't matter that it is triple negative the tumor is so small all you need is radiation. He also gave me an anti estrogen pill to take even though it was not estrogen driven. I asked him about the oncology test and he said i do not need it because if it cam back high he still would not give me chemo because of the size of the tumor. He wants me to go for the BRCA1 & 2 blood test which I am going for next week. I want to also mention before the BC i had uterus cancer which was also caught very early and I i needed was a hysterectomy. Some women on here are asking me why he gave me Arimidex if it was not estrogen driven. He told me he wants to block my body from making more estrogen. He said I have no more cancer I am in remission and the cancer is gone from the surgery. I am so confused first its er+ than after surgery it was neg. I do trust this doctor he comes very highly recommended and has been around for years.