applying occam's razor ( a principle stating that among competing hypotheses, the one which makes the fewest assumptions should be selected ) to this phenomenon, i believe that your original hypothesis is correct.
the medical term for this sign / symptom is dependent edema.
think how you get fat ankles when flying in an airplane for any distance.
think how steve c, and myself and probably more folks, resolved this issue merely by using hiking sticks. their use gets the hands and arms up and moving which in turn helps moves the fluid from the otherwise lower (dependent) and relatively non-moving position.
it is also possible that hyponatremia may play a role.
but for me, in this case, occam works.
