With her age I'd suggest a vet visit.
This might be due to a number of things but one possibility is pancreatic inflammation which you can read about here
http://www.peteducation.com/article.cfm?c=2+1580&aid=335
This could also be due to a bacterial infection, a swallowed non food item or something toxic, a liver or kidney issue, an ulcer etc.
The mustard color to the vomit is likely bile which is produced to help digest food.
As she was seen eating an unknown item this could be toxin exposure or might be a bacterial infection or if it was rodent bait or a dead rodent killed by rodent bait it might be due to that.
Hope this helps you!
An over the counter med you might try would be Pepcid or Tagamet or Zantac.
Be sure to tell the vet when you see them if you use any of these.
You can read about dose amounts (and cautions) scrolling down on the pages here
http://www.petplace.com/drug-library/cimetidine-hcl-tagamet/page1.aspx
http://www.petplace.com/drug-library/famotidine-pepcid/page1.aspx
http://www.petplace.com/drug-library/ranitidine-hcl-zantac/page1.aspx
But with this age dog I'd not delay long before seeing a vet.
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics