Hello! Thank you for the query. A boil on your anus is most likely a hemorrhoid. This condition is caused by constipation, bad diet and lack of physical activity. You may also have itching around the anus and bright red blood on the surface of the stool. Due to antibiotic intake, natural intestines bacteria are destroyed. This bacteria are extremely important for proper bowels function. They protect intestines from bad bacteria. When this good bacteria are gone, bad bacteria are growing what can cause symptoms mentioned by you. I suggest you to have stool tests and stool culture test done at first. Please discuss with your doctor Rifaximine treatment. Hope this will help. Regards.