According to MedlinePlus, a pimple becomes swollen and irritated when the wall of the pore breaks, allowing pus, sebum and debris to leak into the surrounding tissues. This occurs in several types of acne blemishes including papules, pustules, cysts and nodules, according to the American Academy of Dermatology. Common blackheads and whiteheads are contained by the pore and normally do not cause swelling, redness and irritation outside of the pore.
Commonly appearing on the face, shoulders, chest and upper back, acne causes multiple types of blemishes. Most blemishes start out as whiteheads and blackheads. However, the bacteria and pus found within can sometimes move into the skin, causing swollen pimples and deeper blemishes. The American Academy of Dermatology describes papules and pustules as rough blisters that commonly appear close together. Cysts and nodules are deeper and often feel like hard lumps under the skin.
Common blemishes can also become red, irritated and swollen if they are picked at or popped. Not only can squeezing pimples cause the pore wall to break, which ultimately allows the bacteria to enter the skin, the irritation from squeezing alone can cause redness, swelling and pain at the site even when a wall breach does not occur.