If the lump becomes bigger after chemotherapy for breast cancer, it may be caused by the resistance of tumor to chemotherapy drugs or the rapid growth of tumor. 1. Tumor resistance: if the tumor cells are resistant to chemotherapy drugs, resulting in breast cancer chemotherapy not achieving the expected results, the tumor may not shrink after chemotherapy, but increase in size instead, and the lump will become bigger after chemotherapy. 2. Tumor grows too fast: when chemotherapy is administered for breast cancer, chemotherapy drugs have achieved certain therapeutic effect. However, due to the excessive growth rate of breast cancer tumor, although chemotherapy has an inhibitory effect on the tumor, it is not counteracted by the same, and the lump becomes bigger after chemotherapy will also occur. Therefore, if the lump becomes bigger after chemotherapy for breast cancer, it is recommended to cooperate with professional anti-tumor doctors as soon as possible for further examination and comprehensive analysis and judgment, to clarify the reason for the enlargement of the lump, and then consider whether to adjust the subsequent treatment plan.