What could cause ankle and feet swelling?

Many causes. There are plenty of causes of foot/ankle swelling. First, you need to make sure that you are not in acute pain, which could indicate a blood clot. If not, then it could be the venous system not emptying, added salts in foods, lack of calf strength when walking, diabetes, etc. Make an appointment with your local podiatrist who can better differentiate the swelling and recommend a plan for you.
Many things. I suggest you see your dr. For an evaluation.
Swelling. Blood clot needs to be ruled out first. Then you can look at poor venous return, trauma, gout, infection.