The purpose of the study is to evaluate the impact on progression-free survival (PFS) with the combination carboplatin - pembrolizumab in patients with CTC (circulating tumor cells) positive, HER2 negative metastatic breast cancer previously treated with anthracyclines and taxanes. Previous studies have indicated that recurrent breast cancers are more resistant to chemotherapy and maybe associated with a weak immune system. This study is investigating the use of an immune therapy drug, pembrolizumab, that has the ability to restore the capacity of controlling and killing cancer cells of an important component of your immune system called T-cells. Pembrolizumab has been found effective in other types of cancer and has already been approved by FDA for those indications, but the efficacy in breast cancer is still unknown. In this study, pembrolizumab will be combined with chemotherapy to increase the cancer cell killing. There is no control or placebo treatment in this study.