It's probably nothing but get it checked out. I just googled and found this:
Ankle swelling
Ankles can be swollen because of increased pressure in the veins or lymphatics. You can tell the difference between the two because blockage of the veins causes pitting oedema (i.e. if you press the skin with your thumb for a few seconds it leaves a clear imprint) whereas lymphoedema does not. Lymphoedema often runs in families and is impossible to treat effectively.
Pitting oedema due to increased pressure in the veins is treated by working out what causes the increase pressure in the veins! Pressure can be increased because of:
Heart failure
Failure of the muscle pump - for example sitting in an aeroplane for 12 hours, unable to walk and unable to put your feet up.
Allergy - food allergy and chemical sensitivity often causes fluid retention which may manifest as puffy ankles.
Blockage of the veins. If this occurs as a result of a thrombosis in the deep veins this is a serious matter requiring hospital admission for anticoagulation. Typically the lower leg is hot, painful and swollen with pitting oedema. Stretching the calf muscle causes pain. The blockage of the vein is not serious - the problem arises if the clot becomes dislodged and travels in the blood stream to the lungs where it gets stuck to cause a pulmonary embolus.
Oedema is rarely caused by low levels of protein in the blood as seen in malnutrition or liver disease. Obviously this is a serious matter.
Phlebitis
Phlebitis (sometimes called thrombophlebitis) simply means inflammation of the veins. It is caused when blood clots in the superficial veins of the legs. Why it does this I have no idea - I suspect that the wall of the vein gets sticky (perhaps as a result of allergy) and that triggers a clot. Certainly Dr Bill Rea in America has described a group of patients who react to chemicals with phlebitis.
Phlebitis feels like a hard worm in the vein - it is usually painful and often inflamed. It is not dangerous at all because the clot is stuck there and cannot move. It is a bore because it is painful. Doctors often treat with antibiotics but there is no logic to this because there is no infection. Aspirin like drugs are often given to reduce inflammation, but it gets better on its own with time.
The best way to tackle many problems of veins is to lie on your back with your legs in the air. The trouble is this is neither practical not dignified!