It could be something as simple as Crohn's Arthritis. When I'm flaring, my ankles swell to the point I can't walk or drive for 2 weeks. My right ankle is usually the worst.
I know there have been others on here whose knees swell all the way down to their ankles. Others have had their fingers swell.
I'm sure yours could be a NUMBER of different things but I (finally) know mine is simply Crohn's Arthritis (inflammation caused by Crohn's) and nothing more.
It's very annoying and I feel your pain.