There are a variety of reasons for swollen ankles and feet including high blood pressure, cardiovascular problems, circulation problem, kidney problem and something putting pressure on a major vein (the Inferior vena cava) and cellulitis, although both legs at once are less common.
Transient swelling could be related to prolonged immobility causing calf muscles failing to pump fluid back into the circulation. Other causes include varicose veins, side-effects of some drugs and being overweight.
A mild diuretic would be the first action, together with compression hosiery.
Keep your legs elevated at a high angle when you can to encourage drainage and if you have a sedentary job ensure that you get up and move around every hour.
Avoid caffeine drinks, but do continue to drink water. Reduce your salt intake too.
You mention that your doctor gave you keflex, I hope he also listened to your chest and ran some blood tests in the meantime?