Hi there rhaperty,
There could be a number of things going on with your girl here. It sounds as if this "mass" in her throat is compressing her esophagus (tube from mouth to the stomach) and preventig her from eating.
As she is older she could potentially have an enlarged thyroid gland or hyperthyroidism (causing her throat to become enlarged). There are many symptoms of this condition inclduing, increased thirst and urination, increased appetite and also potentially vomiting.The blood work your Vet has carried out will show if she has excessive thyroid hormone in the blood and enable this to be rulled in or out.
She could also potentially have a type of cancer causing her lymph nodesto become enlarged called ymphoma. The best way to diagnose lymphoma is biopsy of the enlarged nodes. However,a full blood and urine panel are always done first to rule out other causes of lymph node enlargement (such as infection) and diagnose any other concurrent disease so the Vet will need to wait for the results before considering biopsy.
There is also a possibility that a foreign body she has swallowed has become lodged in her throat causing localisedswelling of the surrounding tissue.
Whatever the cause here the more pressing problem is the potential for dehyfration if your kitty is unable to keep water and food down. Older cats especially commonlyhave a degree of kidney insufficiency so dehydrate much more easily.
I recommend you do take her to the Vet as soon as you can for fluid therapy (intravenous or subcutaneous depending on degree of dehydration) to ensure she doesn't become dangerously dehydrated while waiting for blood test results and further diagnostics.
I hope this helps, please reply if you have further questions.