Did your surgery produce clean margins? Since you are early stage, there would be no metastases and unless they were unable to get clean margins with the mastectomy for some reason, the chemo is an adjuvant treatment to reduce the risk of recurrence. Conversely, the risk of infection is high, even under the best of circumstances during chemo. If chemo were continued and your re-opened incision became infected, the consequences would be worse, likely resulting in having to be admitted into the hospital again for treatment of the infection and even longer delays in the chemo.
If you and your regular oncologist did not have the "precautions you need to take while in chemo" discussion before you started chemo, you need to have that with him ASAP. Communications and trust are two way streets. If you do not understand the medical reasoning behind your treatment plan, you need to ask.