One of our rescue ducks tonight vomited black liquid - no food in it, and she hadn't been turned out in the yard to forage since about 11:30 this AM, this was at about 10 PM tonight. She has pooped, but won't eat her regular pellets or feed. She was shivering a little bit, but seemed to be upset and stressed when she was separated from the rest of the flock. She is now drinking lots of water, acting happy, and has pooped a couple of times. Still hasn't eaten though. Any advice from duck health experts??