Light Brahma Hen, Weak, Skinny, But Eating/Drinking??

We have a Light Brahma who will be two in April. She has always been healthy, but we noticed on Sunday that she seemed kind of weak and out of sorts (closing her eyes while foraging). Upon picking her up we discovered she is skin and bones. You can't tell by looking at her as she has a full coat of feathers for our NH winters. She is eating (pellets, meal worms, yogurt) and drinking water, as well as eating snow (odd?). She can't seem to make it up to the coop at night, rather laying on the cold sand in the run, so I have been going out after dark and putting her in the coop. I doubt she could stay on the roosts so I just put her on the shavings on the floor (it's a raised coop above a sand filled run). She makes no attempt to get away from me when I pick her up, which is abnormal, she is tame enough, but doesn't like to be picked up. She is the first one at the door in the morning to see what I've brought for them though. I am thinking of setting up a cage in the house for a few days? Any suggestions? We are entering our fifth year of keeping a small flock of hens (6 right now) and we have been lucky with illnesses.

she might have worms, chickens with worms eat, but lose weight

Thanks for the reply! Is the de-wormer something we could pickup at a store like Tractor Supply? We've never had to do this.

I've never dealt with worms, but dewormer is pretty common, so you should be able to find it at TSC

Tractor Supply usually has Safeguard liquid goat wormer, which works for most types of worms. The dose I use is .5 cc for standard sized chickens and .75 for larger breeds, .25 for bantams. Use a needle-less syringe to dose each bird orally. You repeat the same dose 10 days later to get the entire life cycle of the worms. Tractor supply may also have Wazine, but that only works for roundworms. There are also some horse wormers you can use, but you'll have to search the dosages if you use one of those, I don't have them. I would worm all your birds. If one has them then they've all been exposed and likely others will too.
You can have a fecal float test done to be sure if you want, most vets will do it even if they don't see birds. That will ID worm type.
If you have a worm issue then it's a good idea to worm a couple of times a year at least. The worm eggs are shed in droppings and can survive in the soil almost indefinitely. Many people worm spring and fall. My climate and environment is worm heavy so my birds are wormed 3 to 4 times a year. Also there is a recommended egg withdrawl of 14 days following treatment, so with two doses 10 days apart that's 24 days of dumping eggs. It's also a good idea to alternate wormers, to minimize chances of resistance to the meds. Use one type in spring, and another in fall.