Hi Epsilan,
Firstly, have you talked to, or taken Chubby to, a vet? It sounds like she has multiple problems, some of them possibly linked, and the longer you wait the less chance you have of treating them successfully. If you need help finding a good vet, please do let us know roughly where you are - one of us may be able to advise you on reputable vets in your area, or even of a local rodentologist who can help.
The hair loss could be a skin problem such as mites or a fungal infection; it need identifying and treating as soon as possible, as the irritation caused by some skin conditions can be difficult to combat once it gets past a certain stage.
Does her swollen chin feel like it could be a large lump? What does it feel like - soft or hard? Abscesses are common on the lower jaw, and can grow to large sizes and eventually burst on their own if left untreated. However there are other causes of generalised swelling; even mites can lead to such swelling if not treated early enough. Again, a good vet or rodentologist will probably be able to identify the problem there.
The lack of urine and faeces is a concern and is suggestive of no food and water intake. In a guinea pig, this can quickly become serious, due to the way their digestive system is designed to cope with near- constant food intake.
I do not recommend starting syringe-feeding until she has been seen by a vet; there could be a problem with her digestive system, such as a blockage or bloat, which must be ruled out or treated before you attempt to introduce food once again. She will need plenty of fluids, ideally injected (subcutaneous) by the vet. The lack of poops may well be simply a lack of food, but she needs a full checkover to rule out such problems.
The lethargy is a general but serious sign of illness and will not go without her problems being tended to.
Please do contact a vet ASAP. Best of luck, keeping my fingers crossed for her.
