Why does my left ankle and foot swell without pain?

Dependent Edema: Edema means swelling and at the end of the day, the legs/feet are prone to this due to gravity. Edema could be related to water retention which can sometimes be alleviated by water pills - this would need to be examined, but one sided edema may be more related to lymphedema which is an inefficiency in flow of the lymphatic system. Still should be checked but special compression pumps can be used.

Have hypertension?: People with high blood pressure tend to get swelling of ankle and feet at the end of the day especially if stagnant sitting or standing position. Fluid builds up in lower leg because leg has been dependent (hanging down) for long periods.

See below: Could be do to hypertension, or bad valves in the veins creating leaking of fluid and swelling.