What could cause my ankle to seriously swell for seemingly no reason?

Inflammation: One ankle like this suggests an inflammation from gout or an occult infection. Is it red and hot? Elevation and ice compresses might help if it is due to excess mechanical use and in this situation it should be self- limiting. If worsen see a primary care doctor, who can provide you with a differential diagnosis.

Many things: Hypertension, sodium, injury, circulation problems.

Vein problems: Veins have valves to prevent backflow of blood. If they are damaged the backflow causes fluid to seep into the tissues causing the swelling. This usually gets better overnight. If you are diabetic you may have decreased sensation in your ankle. The swelling then may be something more serious. Have it checked out by a podiatrist.

Many things: This problem mandates a real-time (face-to-face) meeting with a doctor. A history has to be taken along with a examination as well as labs and other tests. Only after all this will your doctor be in a position to tell you what's wrong and what needs to be done to help you.