it's very worrying, my old english went off his food the other week, and he eats like a horse normally, he ate something in the garden err a weed or a small stick, probably mooching grass and a weed was ate with the grass, would not eat for three days, then eat did his business out the back, and i could see what looked like undigested twigs i think and a very odd colour mixed with what looked like clear jelly, got better after then wolfed down his food as normal, i hope all goes well for you and you dog.