It is called Orthosthatic Edema. It is leaking of the blood water from tiny capillaries inside the tissues.under the skin in the the ankles due to gravity pressure. Sitting, standing, walking or riding bike etc. long time can cause it. Cutting salt intake , elevating the legs wearing a support hose can bring the swelling down.
To prevent this you should not stay in a dependency position for long time , and change your position once in a while.
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