Don't understand why you did tt your Vet before you posted this. =/
You say, lethargic, nasal discharge, and consumed "dog kibble" in the last week?
NOT normal behavior and it's during your summer, so getting cold isn't an issue.
The digestive system is the most vulnerable in a horse.
You know that dog kibble has meat in it? Horses love to eat it and colic from eating it. Certainly I am careful about what my horses grab to eat, and dog or cat food isn't on the list.