Our (very first) broody hen hatched 8 beautiful chicks who have all been doing really well and are now 3 1/2 weeks old. But, one has been acting lethargic for 2 days, so now on the 3rd day I have brought it inside because on observation I could see that she was trying to cuddle under the hen whenever the hen got near her, which told me that she is cold (weather has been up and down, 40F nights then 75F days, 65F night and 55F day, just all over the place). She's in a bird cage I use as a hospital crate, covered to keep it quiet and dark, with hay bedding like she has in the coop with her mom. I have an infrared heat lamp on her set at 85F, food and water in cups. I have carefully dipped her beak in the water and she'll drink a little but isn't, to my knowledge, going to the water on her own. I have seen her eat a little, yesterday. The hen has been taking them to the edge of the forest and they've been foraging under cover for hours every day, gradually exploring while sticking close to the barn area. They've had chick grit since their first day, are on organic Scratch and Peck Starter. Their water has Avia Charge (McMurry Hatchery product) in it, but they also drink the main water source which has apple cider vinegar in it. Any thoughts? No pasty butt (on any of the chicks, at all), no idea what could be wrong. Doesn't appear to be contagious as all the other chicks are stronger than ever.