This is not all that uncommon. Riding (sitting for prolonged periods), standing for long periods of time, can all cause the blood to pool in the ankles and feet or fluids to collect in this area.
Swollen ankles can have many causes actually. Given the fact that you do not have any health problems (that you are aware of), I will list several things that might be causing your problems. First of all, if only ankle is involved, it could be related to a trauma or injury to that one particular ankle or a disease process like arthritis in that ankle or foot. If both are involved, you will need to consider things like gout or gouty arthritis, venous insufficiency, kidney disease, higher than normal sodium intake, prolonged standing/sitting, arthritis, wearing tight shoes, congestive heart failure, and deep vein thrombus. I would suggest you try to keep your feet propped up and try to stay off of your feet the next few days. Cut back on any high sodium foods that you may be consuming (read your labels) and call your Dr if it is not improving within the next few days or you start developing new symptoms like a warm, swollen calf, trouble walking, shortness of breath, or any other unusual symptoms.
Hope this helps!
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Edited by Nurse Milli on 10/1/2009 at 1:47 AM EST