What causes swollen feet, ankles & legs?

Gravity. Usually prolonged sitting or standing can cause swollen legs and feet, if it improves in the morning after being supine overnight , i wouldn't worry too much , but still see your doctor, its an early sign of venous disease. If it persists see a doctor asap, it could be a blood clot as well.
See below. Swelling of feet and legs could be from insifficiency where valves in veins cannot overcome gravity and blood settles in legs and feet. Lymphedema can also cause fluid to accumulate there. It can also be a sign of congestive heart failure. Elevate feet/legs whenever possible. Elastic stockings and compression therapy may help. See your physician for evaluation to find cause and get treatment.
Edema. Swelling to the feet and ankles is common as we get older. It can be a vascular insufficiency, kidney or blood pressure issue. If you have been given a diuretic by your doctor - take it on schedule! You can try to wear support hose and elevate your fee when out of bed. If this does not relieve your issue see your doctor. Do not ignore this.
High blood pressure. Typically when both feet and ankles are swollen it is due to high blood pressure or some form of kidney block-up. See your podiatrist or vascular specialist for more details and treatment options.