There are several conditions that can cause large hard lumps in the breast during breastfeeding. The most common reason is that during breastfeeding, milk has pooled in the breast, causing stagnation and breast nodules. In this case, it is important to drain the milk and unblock the milk ducts as soon as possible to avoid further stagnation of milk. In addition, if the stagnation lasts for a long time, it may lead to mastitis during breastfeeding, which is also manifested by large hard lumps in the breasts and is very painful to press, accompanied by fever and chills. If breastfeeding mastitis occurs, you should go to the hospital as soon as possible and be given certain antibiotic treatment, and even if a breast abscess forms, you should actively incise and drain it. Another rare cause is breast tumor, which is relatively uncommon during breastfeeding. However, once a person has a baby and is breastfeeding, the hormone levels and distribution may change significantly from what they were before breastfeeding or before pregnancy. Therefore, breast tumor should not be ruled out completely, and it is also necessary to actively visit the hospital. If breast tumor is found, active surgery will be performed.