Swelling occurs whenever the organs, skin, or other parts of your body enlarge. It is typically the result of inflammation or a buildup of fluid. Swelling can occur internally (within the body) or it can affect your outer skin and muscles. A range of conditions can cause swelling. Insect bites, illnesses, or injuries often result in external swelling. Internal swelling is often a side effect of medication or the result of a serious injury.