Following are the causes which can cause edema in foot and ankle area on one side only.
1.Venous insufficiency that is inability of the pumping system of the leg to pump the blood back to heart is the most common cause in elderly.
Other causes include
2.Secondary lymphedema which can be caused by a tumor,radiation, surgery or bacterial infection.
3.Pelvic tumor or lymphoma can cause one leg edema by producing external pressure on veins.
4.Reflex sympathetic dystrophy is a rare cause but is usually associated with injury and has lots of other symptoms.
5.Filiarisis but swelling would not go down on resting.
Other causes which would cause acute swelling include
Deep Vein Thrombosis
Rupture of baker's cyst
Injury to leg
Looking at your history ,venous insufficiency is the most likely cause.A color doppler examination would help rule it out or confirm it's presence.
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Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice.