Sick Duck, what's wrong with her?

[Dances with Ducks]

My seven month old khaki Campbell, who had been laying an egg a day since the end of September laid a rubbery egg yesterday morning. I checked her vent and she had a mild prolapse, not discolored or swollen, that went away in the afternoon. I had noticed for several days someone had smelly runny dark poo that I had started the day after I added extra grains to their Purina layer feed, I removed the grains the next day after the smelly poo started. The poo continued, and today I separated my Khaki in a grassy pen and discovered it was her. I now notice the poo is a little mucousy and dark limey green.

She has been dunking her head in the water a whole lot, and just seems to take an occasional bite of food. At one point I saw her opening her beak and closing it, I thought she was quacking but I couldn't hear because I was inside and the neighbor was running his lawn mower, so I ran out and she wasn't making any noise but still opening and closing her beak. She got up and was making some movement with her neck I had never seen before, don't ask me to describe it, I am so upset I can't even remember now what it even looked like, except I'm sure it was more up and down, or front and in, than side to side. Couldn't detect any egg stuck in there by palpating her tummy. She is pretty heavy set, normally hangs kinda low in the rear, don't think she is any more so today but not sure. Anybody got a clue what's wrong with her?