Vomiting and not eating.

My shepherd mix, Beanie, is 8 years old, weighs about 44 pounds, and has always been very healthy. I feed my dogs Orijen, to which I add mixed (frozen) veggies and two eggs per week, one fish-oil capsule and a small handful of walnuts or almonds daily. He is a good, unpicky eater. Outside, he never eats any acorns, dried poop or anything like that, but he is a grazer. Whenever we are outside, he spends his time grazing on plants and/or grass. I think he just likes the taste because he never, ever vomits.

Until last night, that is. We have two dogs, but I'm pretty sure he was the one who threw up about a 3"x5" portion of his dinner. This morning on our walk he didn't do his normal poking around at everything in the woods, but walked slowly beside me the whole way. He had a normal (no diarrhea) BM. At one point I noticed that spittle was dripping from the side of his mouth and he looked very unhappy. He did not have any nasal discharge. When we got home he wouldn't eat his fish-oil capsule, but ate one walnut and hurried off to get back into bed. His nose is dry and warm.

He's up to date on his shots, is never loose, and hasn't had contact with any dogs other than our other dog in almost a week.

This is longer than I wanted to make my question, but I wanted to be thorough. I'd much appreciate any suggestions you might have about what might be wrong.