A nail becomes ingrown when it grows into the skin, or the skin grows over the edges. Cutting your toenails too short or trimming them too closely on the sides, rather than straight across, can encourage the nail to curl down and grow into the skin. An ingrown toenail may become sore, red, and infected and produce a small amount of pus. Wearing shoes or hose that are too tight or high heels can also set the stage for an ingrown toenail.