What is a big pimple on the middle of your butthip that too sore to touch and red looks infected?

A boil. Boils are skin infections that occur when bacteria enter the skin through a cut or abrasion. They are most commonly caused by the bacteria Staphylococcus aureus. Boils can occur anywhere on the body, but they are most common on the face, neck, underarms, and buttocks. The infection causes the skin to become red, swollen, and tender. A boil may also contain pus. Boils can be very painful, especially when they are large or located in a sensitive area. Most boils will resolve on their own within 1-2 weeks. However, some boils may require treatment with antibiotics or drainage.