Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Bugga is sneezing, and feels congested, eating very little, not producing much urine or stool and sleeping more than usual.
It is very likely that she has picked up an upper respiratory infection. An upper respiratory infection in cats is just like a cold in you and I, and these are commonly caused by a virus, the most common one being Herpes virus. These are easily caught by breathing in virus particles in the air from a sneeze or nasal or eye discharge.
Some cats get sicker than others and young, healthy adult cats seem to tolerate them and fight them off better than very young kittens or older cats.
Lack of appetite and extreme lethargy would concern me, and warrants your watching her very closely. It is understandable that she isn't producing much urine or stool because she is taking in very little food and water. Little intake means little output.
You can help her feel better by adding warm water to her food to make it smell more (they don't eat well if they cannot smell) as well as making it easier to swallow with her probable sore throat. Also the more fluids she gets the better. Offer tuna juice, low salt chicken broth, run the tap if she likes to drink out of the sink.
Take her into the bathroom with you if you run a hot bath or shower as the steam will soothe her sore throat and airways and thin the mucous in her nose making it easier to breathe. If her nose become very congested you can use sterile saline to loosen the thick mucous and remove it. She won't like it but it will help her breathe and be able to smell her food better. You can also use sterile saline to remove eye mucous if it accumulates.
You can give her an amino acid supplement called L-lysine at a dose of 500mg orally twice daily. If this infection is due to Herpes this amino acid interferes with virus replication and will shorten the infection's duration and severity. Good supplements to try are made by the Viralys brand which comes in a powder to add to the food or a tasty gel.
Some lethargy is understandable, let her rest as he needs rest to get better, but if it becomes extreme we need to worry she is developing pneumonia.
If she normally goes outside I would keep her inside until she is back to her normal self.
If you want to try Benedryl (diphenhydramine) to dry up her nose a normal dose is 1mg to 2mg per pound of body weight every 8 to 12 hours. So a cat 8 to 15 pounds could take one half of a 25mg Benadryl tablet every 8 to 12 hours. OR you could try another antihistamine, chlorpheniramine at 4mg per cat orally every 12 to 24 hours. Do not give antihistamines if she has any history of heart disease, hypertension or glaucoma.
If she runs a high fever (more than 104F), has a green or yellow nasal or eye discharge, stops eating completely even with coaxing and clearing her nose and eyes, or starts coughing such that she cannot get comfortable, or having constant difficulty breathing then she needs a veterinary exam. Not an emergency as long as she is eating but soon if she isn't eating or if she has a green or yellow nasal or eye discharge.
Sometimes these upper airway infections turn into pneumonia so that's what we need to guard against.
In most cases antibiotics aren't needed because they can't treat a viral infection, and can contribute to a decrease in appetite so I don't tend to prescribe them unless I feel there is evidence of a secondary bacterial component. Evidence of a bacterial component include a green or yellow eye or nasal discharge, evidence of pneumonia upon listening to their lungs or an infection that lingers without any improvement beyond the normal 7 to 10 days.
I highly recommend testing her for feline leukemia and feline immunodeficiency viruses if her respiratory infection lingers. These immunosuppressive viruses will make a simple infection much worse as they stop the immune system from fighting infections the way it was designed to do.
Other reasons for sneezing and nasal congestion are an infected tooth root, a foreign body inhaled into the nose, a fungal infection, or a nasal polyp or tumor. If she isn't much better in 7 to 10 days she needs a veterinary visit.
Please let me know if you have any further questions.