Bumps on the back of the anus are most likely caused by perianal abscesses and paranal boils. A perianal abscess is an acute septic infection that is bacterial in origin and causes an abscess around the anus with significant pain, which can easily form an anal fistula when the abscess breaks down repeatedly. A paranal boil is also an acute suppurative disease, but it is characterized by a superficial skin abscess that is painful and accompanied by burning and redness of the skin. The patient may have painful and itchy symptoms.