There can be several causes for sweling. Your lack of a known injury should rule that out. Many arthritic conditions can case swelling, but you do not mention any pain, so that may not be the case either. You may have to consult a doctor, as swelling or fluid retention without pain could have any number of causes.
Swelling in the foot, ankle, and leg can be caused by a number of conditions including pregnancy, being overweight, prolonged standing, blood clots, infections, age, heart failure, kidney failure, and liver failure.
Lots of things. Could be an injury to the leg. It could be poor blood flow back up the leg. Meaning the blood gets down there but not back up. Contact MD. especially if it gets worse, change in color or temperature, pain.
You possibly had an edema. It is a condition wherein fluids are accumulating in the affected area. Possible causes includes improper functioning of the kidneys, diabetes mellitus, and any disease affecting the renal system.