If her gums are pale, we need to tread with care. This can be a sign of oxygen uptake issues (less likely if her breathing is normal) but also a sign of anemia. This can make them weak and lethargic but can be secondary to long term infections/inflammation or kidney disease (which can also cause vomiting). As well, further to these, we can see signs of this nature with gut based infections, pancreatitis, parasites/protozoa infections, general dietary indiscretions, and ingestion of harmful items (less likely at her age). As well, cancer, IBD, and metabolic/organ disease could trigger what we are seeing with your lass.
With this in mind, if she is very pale and feeling poorly enough to be withdrawing, then we'd be best to have her seen +/- have a geriatric blood test done at this time. That way we can start narrowing the possible triggers for her signs and counter this before she can loose any more weight. Otherwise, for the moment, we can try to soothe her stomach with an OTC antacid like need Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid), or Tagamet (More Info/Dose Here @ http://www.petplace.com/article/drug-library/library/over-the-counter/cimetidine-hcl-tagamet). Whichever you choose, we’d give this 20 minutes before food to allow absorption. Of course, do double check with your vet if she has a known health issues or is on any medications you haven't mentioned. Though if you give this and she cannot keep it down due to nausea that is usually a red flag that we need to bypass her mouth with injectable anti-vomiting medication from her vet.
Once that has had time to absorb and she is steadier on her stomach, you can consider starting her on a easily digestible diet like boiled chicken, boiled white fish, cottage cheese, scrambled eggs, or meat baby food (as long as its garlic/onion free). There are also OTC vet diets that can be used (ie Hill’s I/D or Royal Canin’s sensitivity) too. The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset. As long as improvement is being seen, I usually advise continuing this until the signs are settled, and then weaning her slowly back to her normal diet.
Since dehydration is a risk if she is vomiting quite a bit (even if she is drinking well), we need to keep an eye on her hydration. To check that she isn't dehydrated, there are a few things you can test. Further to checking for gum moisture, do make sure she doesn’t have sunken eyes and that she doesn’t have a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you are seeing any signs of dehydration already, then that would be our cue to have her seen before this becomes an additional issue for her (especially as it is often dehydration that makes them feel unwell).
Overall, there are a wide range of agents could trigger this GI upset we are seeing. And if this has been chronic as you noted, we'd be worried about underlying organ or metabolic issues for a lass this age. Therefore, we’d want to start supportive care to settle her stomach. If she cannot keep that or water down at any point, appears dehydrated already, or doesn’t respond to the above within 12-24 hours; then we'd want to get her vet involved. They can assess her hydration, check bloods to make sure her organs are working as they should, make sure there are no sinister lumps or bumps in her abdomen that shouldn't be there or any sinister viruses present. Depending on their findings, her vet can treat her with injectable anti-vomiting medication +/- antibiotics or treatment for those above underlying concerns to settle this for her.
All the best,
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