From what you describe it sounds like an upper respiratory virus flare up. You probably know that the common upper respiratory virus in cats is caused by a herpes virus that they contract usually as kittens. Like our common cold sore, it never goes away, it just stays dormant. If something causes a wane in the immune system (any stress event, temperature changes, environmental changes, changes in the cat's routine) the virus can flare up and cause mild symptoms. The virus can cause any of a variety of symptoms, of varying severity. It can cause a mild sore throat, congestion, nose and eye discharges, sneezing, maybe a little lethargy, and loss of appetite. In fact it is often very evident that the cat has an exaggerated swallow, even extending their neck to swallow. They may gag and retch, drool, or gag up phlegm that is often confused with vomiting.
At 13 years old it is very important that Ruby stay nourished and hydrated. This virus usually lasts 3-5 days, often less in vaccinated cats. As long as your kitty is eating some, acting relatively normally with no other symptoms, and there is no color to any eye or nose discharge they require no treatment. If the discharges are yellow or green, they should have antibiotics. If they stop eating, act more lethargic, the symptoms you describe persist, or the congestion and discharge worsens, your kitty should be seen by a veterinarian. Soft, strong smelling foods, if possible, are usually better accepted. I would suggest offering strained chicken baby food to eat. It is soft, easy to digest, palatable, and provides needed fluids.
Hope this helps, we try to answer your questions and concerns as best we can on this site without the benefit of a proper physical examination. Please appreciate that it is always advisable to seek local veterinary care if possible. Let me know if there is anything further I can help you with. Thanks so much, and kind regards!