Swelling on the side of her mouth suggests envenomation by flying insect or spider and fluid under her chin, then, would be inflammatory fluid collecting there because of gravity - a dependent edema. The same scenario could arise secondary to her being bitten by another animal or even from neoplasia if she's an older dog. Salivating and ptyalism (drooling) aren't pathognomonic (specifically indicative) of any one particular disorder but should prompt her vet to thoroughly examine your dog's oral cavity as well as that swelling and edema.
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As alarming as she may look, this set of symptoms doesn't represent an emergency. She's long past the point at which a serious allergic reaction (anaphylaxis) would occur. Please do you best to have her attended to in the morning.
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