Any number of things can cause the swelling. It depends if it is temporary or long term. If it is long term it is probably arthritis. If it is temporary it is probably a sprain or injury. Still, going with 50% probability without knowing more information only gives you the best first guess and it is a wild guess. That leaves a great margin for error.
The common condition is called oedema. It is a fluid build up around the articulated joints and normally associated with long periods of bed rest or immobility.