First, I am glad to hear that a harmful ingestion isn't likely. Furthemore, if we have some sneezing and green mucus (likely nasal discharge but do note that the green is a sign of bacterial infection and antibiotics from her vet would be indicated), then I'd be wary of a bacterial upper airway infection causing all of her signs. As I am sure you can appreciate, this will make her feel unwell and thus lethargic. As well, congestion can often put them off food/water.
In this case, we need to tread with care if she is this young and this depressed and weak. This is because pups have a small body reserve and her not eating is going to drop her blood sugar quite dramatically. Therefore, since she is so down, we'd want to start by giving her a sugar boost. To do so, you can rub karo syrup (or honey, pancake syrup, etc) on her gums to give a boost and get some energy into her.
Otherwise, we need to start some supportive care if she is showing upper airway signs. Since she is sneezing (which is the body's way of trying to push material out of the nasal passage), you can take her in the bathroom while you run a hot shower. The steam will help loosen and clear any material that may be accumulating in the nasal passages. Alternatively, you can also use a baby nebulizer/humidifier, but often dogs don’t like things held up to their faces. Therefore, we can often make little ‘steam rooms’ with her in a small room or big dog carrier with the humidifier near by (you can put a sheet over the crate and humidifier to better focus the steam).
As well, we can clear the nasal passages and reduce sneezing with saline nasal drops (like Ocean Mist or Little Noses but not anything medicated). To do so, tilt the head back and drop two to three drops in one nostril. Again not a pet favorite, but it helps. After the drops go down, you can let the head up and wipe away any discharge that gets loosened. Then repeat with the other nostril.
Finally, making sure she is getting food and water is important, as congestion can make them less then keen to eat. Therefore, do try tempting her with soft wet foods (since they are high in water). As well, if she cannot smell the food properly, you can also consider warming it up a bit in the microwave to help her with that. Otherwise favorites or bland diet options like rice with boiled chicken, white fish or scrambled egg can be offered. And I would also note that if she isn't vomiting, we can syringe feed her. We can use calorie rich diet like Hill's A/D, Royal Canin Recovery, Clinicare Canine/Feline Liquid Diet or Dogsure. But as well, we an water down canned puppy food to syringe in a pinch. All of these are nutrient dense, so a little goes a long way nutrition-wise. And these could just help get some more calories and energy into her to help fight this.
Overall, that green mucus you found is a sign of bacterial infection and that will make her feel poorly. That said, given her signs and the sneezing, that green mucus is likely from her upper airway. Therefore, we'd want to start the above now to get her blood sugar up and try to get her comfortable. But once her vet is open, we will want to have a check since that green is a clue that we are going to need antibiotics to get her settled here and back to feeling like a pup.
Please take care,
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