That could be caused by several things, including low protein levels in the blood, some degree of heart failure, potentially fluid around the heart (a pericardial effusion), or just general illness leading to leaky blood vessels. Also, sometimes the edema could be from the lymphatic vessels not draining properly because they're partly blocked by enlarged lymph nodes, or a blood clot in a leg (deep vein thrombosis, or DVT), although the last two possibilities are usually associated with only one leg having swelling.

Obviously, there's a wide range of possibilities, some very minor, and other more significant. It helps to have a doctor directly evaluating the situation.

Good luck.

-Dr. West