I’d like to help with your concerns about Patches's lethargy, and lack of an appetite.
You are right to be concerned as a guinea pig that doesn’t eat well is in big trouble. Their gastrointestinal system needs to have food constantly moving through it or normal motility will be lost and there will be overgrowth of bad bacteria.
If she is refusing to drink water as well that is very worrisome too as they can quickly become dehydrated.
I suspect her lethargy is a direct result of her not eating and drinking and a reflection of how sick she is.
It isn’t possible for me to know what started all of this without physically examining her but possible causes include vitamin C deficiency, which leads to painful joints and poor mucous membrane health which can make eating and chewing painful, a respiratory infection, overgrown teeth or spurs on the teeth, especially the premolars and molars which can develop painful spurs over time that make eating painful, or metabolic diseases like kidney disease or diabetes, or even cancer.
At her age overgrown or abnormally worn teeth are probably the most common problem we see.
But my concern right now is also with her gut. She needs to eat and she especially needs fiber to get her intestines moving properly.
Make sure to have fresh timothy or mixed grass hay available to her at all times.
Because she won’t eat on her own I highly recommend force feeding her. You can use a medicine syringe and feed her ground up pellets mixed with water or orange juice or vegetable baby foods (carrots squash and green beans seem to work best). If you can find it Oxbow critical care for guinea pigs is even better as it is a balanced diet that can be force fed. Oxbow critical care can be purchased through your veterinarian or some pet stores may carry it. You will need to force feed every couple hours during the time she’s awake.
I also recommend getting her some probiotics to replace the good bacteria. Nutramax company makes Proviable, which is a pretty good product. If you had another healthy guinea pig available you could make a slurry of his or her night droppings and sterile water and give her that. These droppings are rich in beneficial bacteria and trace elements and vitamins essential for good health.
If it is cold where you are put her in a travel cage with lots of bedding to snuggle in or place a heating pad under part of her cage set on low to give her warmth. We don’t want her wasting too many calories just trying to keep warm.
Then do your best to have her seen by a veterinarian that is familiar with guinea pigs. I’m afraid finding one tonight may be very difficult. Here is a link to help you find a guinea pig veterinarian tomorrow monring:
If there isn’t one close to you then call local veterinarians and ask about their experience with guinea pigs. Here is a link that will give you tons of information about guinea pigs:
Please let me know if you have any further questions.