Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Biscuit was weak last weak, had hives (bumps all over) and was vomiting and now since last night is vomiting her food and then yellow, bile material and refusing to eat.
Yellow in the vomit means that the small intestine is refluxing bile into the stomach so that when she vomits you see the yellow color. That isn't normal as bile doesn't belong in the stomach, and it does mean that there is some reverse motility, but it isn't specific for any particular disease process.
In many cases vomiting is triggered by eating something they should not, too much table food, too many treats or something they find outdoors, esophageal reflux, or a dietary allergy or sensitivity.
More serious causes of vomiting include viral or bacterial infections, chronic pancreatitis, inflammatory bowel disease, internal organ failure (kidney or liver disease), a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
Rarely a parasite called a stomach worm (Physaloptera) can lead to intermittent vomiting. These are difficult to diagnose because they rarely shed eggs, so we can try simply worming her with a gentle wormer called Pyrantel and see if that helps. This is easy to do and inexpensive so well worth trying.
My concern with her is that this seems to be happening repeatedly. So either she is finding something to eat outdoors that she is allergic/sensitive to (that would fit with hives too) or is causing her stomach upset.
For now until we figure this out it may also be helpful to put her on an acid reducing medication as too much stomach acid, especially on an empty stomach, is very irritating and predisposes to vomiting.
You can give either:
1) Pepcid ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 20 to 40 pounds of body weight every 24 hours
These are both acid reducers and should help her feel better. They are quite safe and can be used for several days if necessary.
If there is no vomiting for 12 hours offer a bland diet of 1/3 boiled, minced, white skinless chicken or boiled, lean hamburger and 2/3 boiled, white rice mixed with some low salt chicken or beef broth to make it easy to lap up and swallow. If she refuses that you can offer a little meat baby food. If she refuses both then don't push it, she needs veterinary care.
But if things go well and she does eat and doesn't vomit feed her the bland diet for 2 to 3 days then slowly start to mix back in her regular food, a little more at each meal. It should take about 5 to 7 days to slowly convert her back to her regular diet.
If she continues to vomit even with the acid reducers, runs a fever (more than 103F rectally), has a tense painful belly or if she refuses to eat she should see a veterinarian for an examination, and some diagnostic testing. I would start with a complete blood count and biochemistry profile, a specific test for pancreatitis called canine specific pancreatic lipase, as well as fecal checks for parasites.
Please let me know if you have other details or a particular question based upon my response.