The pain in the lower left side of the abdomen may be caused by acute enteritis, ulcerative colitis, left-sided adnexitis, ectopic pregnancy, etc. The pain in the lower left side of the abdomen may be caused by acute enteritis. 1. Acute enteritis: Acute enteritis may be triggered by eating unclean food or catching a cold in the abdomen, as well as intestinal infections, etc., which may be accompanied by diarrhea, and the symptoms will improve significantly after the bowel movement. 2. Ulcerative colitis: When ulcerative colitis is in the left half of the colon, mainly in the rectum and sigmoid colon, it will cause pain in the lower part of the left abdomen, which may be accompanied by mucus and blood in the stool. 3. Left adnexitis: it may be caused by endometritis which is not treated in time and the inflammation spreads upward to the left adnexa. 4. Ectopic pregnancy: the fertilized egg will seriously block the fallopian tubes if it is deposited outside the uterus, and if the left fallopian tube ruptures, the pain in the lower part of the left abdomen may occur. Ectopic pregnancy should go to the hospital immediately for treatment to avoid life-threatening situations. There are more reasons for women to have pain in the lower left abdomen, so they should go to the hospital for relevant examinations in time to clarify the cause and treatment.