Need more info. More information would be needed to try to understand what might be going on. I'd recommend you start by seeing your primary care physician to an initial evaluation. They are many possible causes of swelling in one leg. It is very important to have this evaluated. If the swelling is sudden, i'd recommend you be seen immediately.
Many causes and RX. Leg swelling is a common complaint with many causes including problems with the heart, lungs or kidneys, as well as medications, lymphedema, dvt, cellulitis, venous insufficiency and a host of other issues. There is also a difference in cause if the swelling is one side or bilateral. There are many treatments for swelling but the first step is to identify the cause. See a vein specialist or md.