The problem of lower abdominal distension after sex is different for men and women.
Women may have gynaecological inflammation, such as pelvic inflammatory disease, cervicitis, adnexitis, endometriosis, etc. They can go to the gynaecology department for an examination to clarify the cause through physical examination and gynaecological ultrasound. If it is a male, it may also be caused by prostatitis or urinary tract infection and can be examined at a urology department.