How come I have a huge pimple on my back?

Natural Process. Pimples occur when glands in our skin get blocked and what they secrete collects and causes pressure. The immediate response is inflammation (red, hot, tender swelling). This is the body increasing the blood flow to the injury to bring healing factors to the area and also the veins expand to carry bad stuff away. Gentle cleansing and avoid picking/squeezing lets nature take its course.