Won't eat, vomits, whines, restless

I have a 8 year old boxer. Never had any health issues other than ear infections. On Monday he started vomiting every once in awhile; usually clear mucus with red or brown specks in it. He went on a walk, was fine. Then suddenly on Tuesday, he looked like he felt bad, stopped eating, and throws up every time he stands. Now the vomit is darker in color almost black, with dark specks. I took him to vet and they said that he has an infection but could not determine what was the cause. He still has not eaten, and he continues to vomit every time he walks a couple of steps. He does drink water, but that's it. He is really restless when he goes to lay down. His leg twitches about every 30 sec or so, and he can't seem to get comfortable. He looks like he tries to stretch out his stomach while laying down, or lays on his side with his legs open. I noticed he has a hard time curling in a ball like he normally does  and he starts to whine when he tries to pull his legs in. How can I get him to eat something. Is there anything specific anyone can possibly tell me. How can i get him comfortable. Does it sound like he has possibly eaten somehting that he just can not digest or like something is possibly stuck inside?

[Aleda M Cheng, D.V.M., C.V.A]

What you describe could be a Bleeding Gastric or Duodenal Ulcer, Hemorrhagic Gastroenteritis, Pancreatitis, Foreign Body Ingestion with or without an obstruction, Cancer, Toxin ingestion, and others.  Could your dog have accidentally ingested someones prescription medication?  

Black or very dark vomit could be due to digested blood in the vomitus signifying a bleeding ulcer.

Please bring your dog back to a vet.  Allow your vet to perform X-Rays with or without a barium study, and ultrasound, if necessary.  Bring a sample of the vomit.