many times overly moist soil will result in leaf drop. are you letting your begonia dry out between watering, and how heavy is its substrate? if you have a few green leaves, take a couple and root them as insurance if your begonia is a rhizomatous, scandent or shrub type. remember: begonias don't like wet feet.
in the mean time, remember that winter is a season of low growth. you might want to hold off on too much water and definitely don't fertilize if you see that your plant is slowing down. what might help in diagnosis is a picture of your plant and its set-up.

-ming