My senior dog eats gravel

My 15 yo Corgi-Beagle female mix suddenly lost a LOT of weight. She stopped smiling, sleeps most of the day, and goes in the backyard at 2-3 am. She eats very fine gravel around a flat podium with rubber top we built for her under the deck, to be shielded from rain and sun. Does she lose weight because she is sick or because she is stuffed with gravel in her guts? Also she started hiding in the corners.