Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I'm so sorry to hear that Chelsea Mae isn't feeling well.
This does sound like an upper respiratory infection, these are often viral and there are some things that you can do to make her more comfortable.
You can help her feel better by adding warm water to her food to make it smell more (they don't eat if they cannot smell) as well as making it easier to swallow. That will also get more fluids into her. The more fluids she gets the better. Offer low salt chicken or beef broth to drink, or give her ice cubes to lick if she enjoys these.
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 respiratory tract making it easier for her to breathe. You can use a humidifier, especially in the room she sleeps in, to help her breathe as well.
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.
Some lethargy is understandable, let her rest as she needs rest to get better.
If you want to try Benedryl (diphenhydramine) to dry up her nose a normal dose is 1mg per pound every 8 to 12 hours. So a dog 8 to 12 pounds could take one half of a 25mg tablet every 8 to 12 hours. OR you could try another antihistamine,
chlorpheniramine at 4mg per dog orally every 12 to 24 hours.
If she runs a high fever (more than 104F), has a green or yellow nasal or eye discharge, stops eating even with coaxing and clearing her nose and eyes, or starts coughing or having 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 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 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. These include a green or yellow eye or nasal discharge, evidence of pneumonia upon listening to their lungs or an infection that lingers beyond the normal 7 to 10 days.
Other reasons for sneezing and nasal congestion are an infected tooth root, a foreign body inhaled into the nose or a nasal polyp or a fungal infection. If she isn't much better in 7 to 10 days she needs a veterinary visit.
Let me know if you have any further questions.