If there is pain in the small abdomen and anal swelling during menstruation, the possible causes include the following: 1. rectal and perianal lesions: such as anal fistula, perianal abscess, or may be due to hemorrhoids, especially when hemorrhoids are combined with infection or thrombosed external hemorrhoids are more likely to occur. 2. rectal cancer or anal canal cancer: can also lead to pain in the small abdomen and anal swelling. 3. pelvic effusion: pelvic effusion may be due to gynecologically related inflammatory lesions, such as adnexitis, etc. If you find pain in the abdomen and anal swelling during menstruation, it is recommended that you go to a regular specialist hospital to find an anorectal surgeon and a gynecologist to improve the relevant examination and make a clear diagnosis, and then provide timely symptomatic treatment according to the different causes.