I am so sorry that you have had such a long wait to get a response, especially when you have what sounds like a very sick dog. I see that you are offline now, and as I have to go into some appointments, I am going to give you my thoughts based on what you have told me so far about your dog.
There are a number of different problems that could be causing the symptoms you are describing. The ones that I am most concerned about are blood loss into her belly, congestive heart failure and hepatitis. I will tell you a bit more about each.
1. My biggest concern is that your girl may have a Hemangioma (non-cancerous) or Hemangiosarcoma (cancerous). These are growths on the spleen that can cause internal bleeding into the belly if they rupture. So, you can't see the bleeding, but they are still losing blood. If there is a sudden, profound blood loss the symptoms are lethargy and thirst, pale gums, sudden collapse and fluid in the belly. If it is more gradual, you might see just weakness and wobbliness, with an increase in drinking. Here is more about splenic masses:
This can be diagnosed by blood work and x-rays and can be treated by surgical removal of the spleen. If the bleeding was caused by a benign hemangioma, then this is curative. If it is a cancerous hemangiosarcoma, it may be curative if the tumour has not spread. If it has spread, most dogs will succumb to it within 6 months.
2. A common reason for rapid, shallow breathing is congestive heart failure. In an older dog, this could be caused by a few different things, but mitral valve disease would be the most common. Basically, when the heart beats, not all of the blood inside it is pushed forward as the mitral valve often becomes "leaky" in older small breed dogs. So, now you have more blood coming in, PLUS that leaked blood that didn't go out, so the pressure builds up inside the heart. This then backs up so there is increased pressure in the blood vessels in the lungs. Because of the increased pressure, fluid gets squeezed out of the blood vessels and into the lungs. With water in the lungs, it is very difficult to get oxygen across the lungs into the blood stream.
Here are some links to more information about congestive heart failure (CHF):
This disease is something which needs prompt veterinary attention. Your dog may need injections of a diuretic (such as Lasix/furosemide) to quickly move the fluid out of her lungs so she can breathe more easily. The vet would likely get x-rays of the chest to see how much fluid is there, and what size the heart is.
Once she is stablilized, she would likely go home with Lasix in pill form so that the fluid doesn't build up again, as well as something called an "ace inhibitor" to help the heart function more effectively.
3. Another possibility is that she may have a liver problem. With liver disorders, the urine can be bright yellow. Often, the sclera (whites of the eyes) will turn yellow, as will the insides of the ears. As well, the gums and soft palate will turn yellow, but this only happens later in the course of the problem.
One liver problem is hepatitis (chronic active hepatitis, CAH). This can cause vomiting, loss of appetite, drinking more and urinating more, jaundice (very yellow urine, yellow in sclera and mucous membranes), and lethargy. Dogs with this may need hospitalization if it is severe. Intravenous fluids may be needed for rehydration. Dogs often need antibiotics, anti-inflammatories, and liver specific drugs to help it to function.
Here is more about it:
So, as you can see, the symptoms you describe could be indicative or a number of different possibilities, some completely curable (hemangioma, hepatitis), others only treatable (congestive heart failure, hemangiosarcoma). Some carry a great prognosis, and others a poor prognosis. The place to start would be a complete blood and urine analysis and x-rays. At my clinic that would cost about $400.
Based on the results of these tests, a veterinarian would be able to determine what is the most likely thing going on with your dog. I really feel there is SOMETHING going on with her, and am concerned that it may get much worse over the course of the next few hours. I do recommend that you take your dog to see a veterinarian immediately to find out what is going on and get started on treatment. If your vet is not open, you should take your dog to see an emergency veterinarian. I hope that with treatment she will be feeling better soon!
If this has been helpful, please Accept my answer and leave feedback. I will still be here to provide further information if you need it!