Now even though she didn't react to you pressing on her belly, the intermittent yelping and increased breathing has me concerned she was putting on a brave face during your palpation and that these are being triggered by pain. Given her other signs, this could be related to a stomach ulcer (primary or secondary to the anorexia or something sharp if she did ingest something she should not have), but can also be due to gut occlusions (from non-edible items but also tumors or compression from enlarged organs at her age). As well, we can see signs of this nature associated with inflammation or infection of the pancreas. Furthermore, while inflammation/infection associated with any of these can increase the thirst, we'd also have to be wary of metabolic (ie diabetes, cushing's disease, addison's, etc) and organ (ie liver, kidney, heart) issues as all can impact drinking as well.
Now in your lass's case and with all her signs, I do think it'd be ideal to have her seen urgently. If she were my patient and you phoned me on the holiday, I would see her at this point. This is because we are best to at least get the pain treated (which we don't have any OTC safe options given her lack of appetite) +/- start her on anti-nausea and appetite stimulating medication to get her eating for us.
Otherwise, if there is any delay in getting her seen, I would suggest that you can at least try to offset stomach upset putting her off her food by treating with an antacid. There are a number that are available over the counter and pet friendly. I would advise only treating with one, but the ones I tend to use are:
*Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid)
*Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac)
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 offering food to allow absorption. Of course, do check with her vet before use if she has any known health issues or is on any medications you didn’t mention.
Once that is on board, we'd want to tempt her to eat (as I am sure you have). You can try her on a light diet option like rice with boiled chicken, white fish, scrambled egg, or cottage cheese. Or since we don't have any vomiting here, we can even syringe feed her using watered down canned puppy food, Dogsure, Clinicare, or veterinary calorie rich diets like Hills A/D or Royal Canin Recovery diet. All have more nutrition per bite and can get some calories in even if we cannot get a large volume in.
Overall, I am quite concerned about your lass. I am concerned that we have a gut based issue that is causing her pain. Therefore, it would really be best to have her seen at this stage if possible. That way we can get to the root of which of the above issues is lurking, address it and get her pain free and eating as quickly as possible.
Please take care,