I understand that you are concerned because Precious has discharge from her left eye and swelling under her left eye as well.
Is the swollen area on her cheek in the area just below her eye? (see the picture of the dog in the link below)
If so given her history of losing many of her teeth and a decreased appetite the most likely cause of her symptoms is actually a premolar tooth root abscess that has fistulated through the bone and into the tissue under her eye skin. The roots of the upper premolar teeth end up just below the eye and if the tooth becomes infected the infection often travels through the soft tissue around the root and drains out through the face. As the infection worsens we can see quite a bit of swelling on the face. The tooth crown may look normal in the mouth or even be missing, as it is the root that is infected, but sometimes we can see how the infection entered via a crack or chip fracture by looking at her teeth.
The infection and irritation in the tissue below her eye would lead to her eye discharge.
The other common cause of sudden swelling and redness is an allergic reaction. This can be from an insect bite or sting, pollen, or plants (much less common in dogs). Sometimes we never figure out the cause of the allergic reaction.
If this is an allergic reaction treatment is based upon stopping further allergic reaction with antihistamines like Benadryl and sometimes cortisone if the reaction progresses.
You can give her Benadryl (diphenhydramine only don't use the combination products with decongestants or acetaminophen as they can be toxic for dogs) at a dose of 1mg per pound of body weight or one half of a 25mg tablet per 10 to 15 pounds of body weight orally every 8 hours. You'll need to give it for at least 72 hours (3 days) to allow the allergen time to clear. If you stop too soon the reaction will recur.
Dogs take a really high dose compared to people, so don't let the amount worry you. They require more than we do.
This may make her a little sleepy. And then you'll need to watch her closely for any progression of symptoms. That includes difficulty breathing, vomiting or diarrhea.
The swelling should resolve slowly over the next 12 to 24 hours.
If her swelling resolves then you know this is an allergic reaction. Because you know now that she has these types of reactions you should keep Benadryl on hand at all times.
Other possible reasons for what you are seeing other than an abscessed premolar tooth root, include an infected or blocked salivary gland, a type of cancer in his lymph node (lypmphoma) or a bite or scratch wound that has become abscessed/infected. If your girl isn't improving with antihistamines then she needs to see her veterinarian for some testing to determine why she has this swelling, and appropriate treatment.
In the meantime flush her affected eye with sterile saline and apply artificial tears to soothe her eye every couple of hours when she is awake.
This isn't a true emergency as long as he is still eating and drinking and the eye isn't worsening.
But she should see her veterinarian next week.
Please let me know if you have any further questions.