What could cause swelling in the feet and ankles
Lots of things. If you haven't recently done something to injure your ankle (twist it, or turn it) then you should get it checked out. Check for some bruising, if there isn't any, and you can't remember twisting it you should really put a rush on getting it checked out. Otherwise try RICE(rest, ice compression, elevation), and if after 2 days the swelling still has not gone down go get it checked out.
Pregnancy can do that.