There are several possible reasons for the vague pain in the abdomen 20 days after childbirth. First, the contraction of the uterus after childbirth can cause the vague pain in the small abdomen. Some female patients do not have good contraction of the uterus after childbirth and do not recover well, so this pain can occur. Secondly, the vaginal infection caused by sexual intercourse after delivery or other conditions may cause vaginitis or pelvic inflammatory disease. Third, there may be fluid in the pelvic cavity, resulting in pain in the abdomen. Fourth, after delivery is not residual things in the uterine cavity, resulting in puerperal infection, which can also lead to small abdominal pain. The mother is required to go to the hospital in time for a gynecological ultrasound to see how the uterus recovers and whether there is residue in the uterine cavity. In addition, the mother is asked not to have sex within a month after giving birth and to keep her vulva clean and her underwear clean.