I am sorry your kitty is having trouble. I am worried that your kitty is going to get into some pretty serious medical trouble because she is not wanting to eat or drink as much as normal. Cats need to eat, and are at very high risk of liver failure if they do not take in enough calories. I suspect that your kitty is nauseated because of the symptoms you are describing and I think that this nausea is due to an illness that has yet been diagnosed. Cats are considered to be geriatric at 8 years old, and are prone to illness as they age. The 3 most common ailments known to afflict elderly cats are diabetes, kidney disease and hyperthyroidism. The symptoms include vomiting, diarrhea, nausea, dehydration, weakness, muscle wasting, poor hair coat, weight loss, changes in appetite, changes in behavior, increased water consumption, increased urination, and increased vocalizations. Unfortunately, antibiotics are most likely not what this kitty needs and a 3 day course would not be all that effective if they were needed. This kitty needs to see a vet, and it needs to be soon. Once cats are exhibiting signs of illness, they are often experiencing advanced stages of illness. Your kitty is going to continue to decompensate and dehydrate. If you are able to have her seen now, you may be able to avoid hospitalization and expensive vet bills. There are no safe over the counter medicines that can help your kitty at this point, and she really needs to have some bloodwork and a urinalysis done to screen for the above illnesses. I know you are isolated from good veterinary care and money is an issue, by your kitty really needs medical attention. You can apply for care credit, which is a veterinary credit card accepted by most hospitals.
Please let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. Best of luck with your kitty!