In someone your age who is suffering from swelling of just one leg, the first and most important thing is to make sure you are not suffering from a blood clot in the leg, called a deep vein thrombosis. Both your age and weight put you at risk for this and, even more importantly, smoking greatly increases your risk. It is important to sort this out, as a blood clot in the leg can be dangerous, as it can break free and lodge in the lungs (called a pulmonary embolism), resulting in trouble breathing.
I suggest showing this as soon as possible to your primary care doctor
, who may decide to perform an ultrasound
of the leg to look for a blood clot.
If a blood clot is ruled out, then other possibilities would include trouble with your heart, which usually causes swelling in both legs but can occasionally mostly affect one leg, trouble with dilated varicose veins
in the leg leading to swelling, or an inflammation of one of the joints in the foot or ankle (such as gout).
Regardless, the first and most important step is to call your doctor and set up an appointment to determine if further testing is needed.