If a patient has left-sided back pain during breastfeeding, it may be related to the following reasons: First, it may be caused by muscle spasm in the left lumbar region. When a patient is breastfeeding, he or she may often adopt a right-sided lying position to breastfeed the baby, which over time may cause muscle spasm in the right shoulder and left lumbar region, leading to pain. Secondly, it may be caused by lesions related to the urinary system on the left side, such as kidney stones or urinary tract infection on the left side. When the patient is breastfeeding, due to the need for prolonged bed rest, less water and continuous breastfeeding, it is easier to develop the above-mentioned lesions in the urinary system, which may cause left back pain if it develops on the left side. Thirdly, it is also common for patients to have an attack of lumbar disc herniation, when the patient may have a long time of poor sitting or lying posture, followed by the onset of such lesions, which may also cause pain on one side of the waist.