Hi Jacqueline,
While every diagnosis is different, I also had TNBC with a tumour that was 18mm, but no lymph node involvement. I opted for a bilateral mastectomy but then had 5 months of chemo, although no radiation required (due to full mastectomy and absence of cancer in the lymph nodes). I was told that while I should consider myself cancer-free after the surgery, due to the aggressive nature of TNBC, chemo was required. Now, your tumour may have been very small and well isolated so that the chemo isn't required, but I would have thought that a question to the oncologist or breast surgeon would be totally justified. If you are still unsure, ask to be referred to another oncologist.
After all, it is your body and while I wouldn't wish chemo on anyone, it was my understanding that it was my best insurance policy!
Take care and hopefully your case was such that the risk of recurrence is so very small that they thought that the potential chemo side effects outweighed any potential benefit.
Best wishes,
Alison