NO pain meds - unless avian specific pain meds by a vet. human or any mammal based pain meds can potentially kill chickens and any bird. even neosporin with pain reliever can't be used on chickens.
be more specific - how long has this behavior been going on? how old is the chicken? Poop - normal, runny, watery, foul, tarry, etc ...
respitory signs? (eyes, nose, breathing )
Physically check her over - feel all over her. look for wounds of any kind.
the signs shes showing aren't typically what you'd see in a bird for pain - birds do not typically show pain - they hide it very well - had one with a missing toe walk around with the slightest limp right after losing the toe. Had a rooster who got a freak puncture in his chest (never figured otu where he got it) go 3-4 days before I figured out he had a injury (only because I happened to feel it) - they are 'food' animals - and as such tend to keep pain or injury or illness secret and hard to tell.
Your hen sounds more like typical stress or even shock symptoms. - first thought would be trauma/injury. second thought is egg bound or internal layer - if she laid a soft shell internally or any egg and it broke - your not going to feel a solid hard egg.
you need to figure out what is causing her to act like this. only real treatment you can give without knowing specific cause is adding vitamins/electrolytes to water and if you want give a dose an antibiotics.
