A few of my hens were lethargic too. I think sometimes an egg breaks inside of them and they don’t eat, drink, and their poop is liquid. So maybe check for liquid poop next time it happens, (hopefully it doesn’t) and what I did for this specifically is put a drop of oregano oil in their water...
I put one drop of it in a gallon of water for my chickens every time I have to change their water. They don’t seem to notice the different taste. Feel free to use it whenever. (I use it everyday. It prevents illness and keeps their insides clean.) But definitely use it when they are not feeling...