There are many reasons why menstrual periods can cause pain and swelling in the abdomen. When menstrual blood passes through the fallopian tubes and reaches the pelvic cavity, there is a transient accumulation of fluid, so that the menstrual blood will stimulate the uterus and cause small abdominal distension and pain. The contraction of the uterus during menstruation helps to peel off the endometrium, which can also cause painful swelling in the small abdomen due to dysmenorrhea. When too much prostaglandins are released in the body, it will not only cause confusion in the function of the gastrointestinal tract, but also cause small abdominal distension and pain.