The bumpiness and breakouts are caused by a condition called Keratosis Pilaris, or as its commonly referred to, "KP".
What exactly is KP?
During the lifetime of normally functioning skin cells, they're born, live for a while at the bottom of the outermost layer of our skin, and just when more new cells now start to push older cells up and out of the way to form the next layer, these cells slowly start to produce and accumulate keratin protein granules. As more and more protein granules accumulate inside older cells, they take on a slightly bumpy exterior.
Continuing to move up closer to the surface, these protein-stuffed cells start to flatten and smooth out, and various waxes, fats, and sugars glue all these cells together to form the brick-and-mortar-wall-like top layers that make up the protective, slightly acidic barrier we call the Acid Mantle. These days, you might hear it referred to as "the skin's natural barrier".