Hello! Thank you for the query. This ling brown string can be a blood, undigested food or parasite. As you have also rectal pain, most likely this is blood string as you may suffer from rectal fissure or hemorrhoids which has become clotted. However rectal pain sometimes can be caused by other pathology like rectal polyp or even colon cancer. That is why I suggest you to have at least rectosigmoidoscopy (or colonoscopy). Stool tests should be also performed. For fissure treatment you should keep your stool soft (increase fiber rich products and mineral water intake, avoid fatty foods), do not push hard on stool, use prescription free suppositories for hemorrhoids. Rectal area should be kept clean and dry. Wash your rectum after every stool. You may ask your doctor about Nitroglycerin gel. Sitz baths with Quercus Cortex can help as well. Hemorrhoids can be treated same way excluding Nitroglycerin gel. Hope this will help. Regards.