Feeling a hard lump in the left lower abdomen lying flat may be caused by enlarged lymph nodes, fibro-lipoma, sigmoid colon cancer and so on. 1. Enlarged lymph nodes: Lymph nodes are part of the body’s immune system, and there are more lymph nodes in the area of mesentery. If there is inflammation due to some reasons, such as bacterial infection, viral infection, etc., there will be enlarged lymph nodes. 2. Fibro-lipoma: most of fibro-lipoma occurs on the body surface, but some of them occur in the abdominal cavity, chest wall and other parts of the body, this is a kind of benign tumor, if there is no obvious clinical symptom, it doesn’t need special treatment. 3. Sigmoid colon cancer: sigmoid colon cancer is a type of colon cancer, which is a kind of malignant tumor occurring in the sigmoid colon, and the symptoms are not especially obvious in the early stage, which is easy to be neglected. However, with the development of the disease, patients will have symptoms such as change of bowel habit, blood in stool, etc., and also have abdominal mass, which is usually hard and nodular with certain mobility, and if the cancer penetrates and infects the mass, it will be more fixed, and there will also be obvious pressure pain. It should be noted that a hard lump lying on the left lower abdomen may also be some other diseases, which should be examined in detail in the hospital, and the cause of the disease should be clarified for symptomatic treatment, so as to avoid delaying the condition.