Pustules may form when your skin becomes inflamed as a result of an allergic reaction to food, environmental allergens, or poisonous insect bites. However, the most common cause of pustules is acne. Acne is a skin condition that develops when the pores of your skin become clogged with oil and dead skin cells. This blockage causes patches of skin to bulge, resulting in a pustule. Pustules usually contain pus due to an infection of the pore cavity. Pustules caused by acne can become hard and painful.