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Does your cat like to chew on cords?
Did you use any flea or tick medication on your cat in the last day or so?
Does your cat take any other medication at all?
What you are describing is nausea in cats. Cats who get nauseous will often drool and be lethargic. Some will vomit, some will not and just continue to drool. I would be worried enough to have him examined in the morning but if he does not worsen beyond drooling and being lethargic then it's likely not an emergency.
I hope this helps, best wishes.