Many. From heart failure, kidney, liver failure, venues insufficiency, venous obstruction, blood clot in the legs, and drugs, and lyhatic obstruction. You will need to be examined and your medical history reviewed. Talk to your family doctor soon.
Long list. Age, weight, trauma, heart trouble, blood clots?
Commonly dependent status with legs down and any history of venous disorders or phlebitis, DVT etc,
Heart failure and fluid overload can occur also.
Bunch of other things!