Sometimes, the hair, sebum, and skin cells clump together into a plug. The bacteria in the plug cause swelling. Then when the plug starts to break down, a pimple grows. Below are the stages of acne and types of pimples: Whiteheads. These are pimples that stay under the surface of the skin. Blackheads. These pimples rise to the skin’s surface and oxidation makes them black. Papules. These are small pink bumps that can be tender. Pustules. These pimples are red at the bottom and have pus on top. Nodules. These are large, painful, solid pimples that are deep in the skin. Cysts. These deep, painful, pus-filled pimples can cause scars.