That sounds like she may be eggbound, they waddle around looking miserable, appetite off, plus frequent visits to the nest box - if her abdomen is hot that could mean an abdominal infection most likely linked to an egglaying dysfunction.
Can you feel a lump under her vent and between her legs? Be very, very careful handling her as there is a risk of breaking the egg. This is what you can do in the first instance if you think she is egg bound, and it wont do any harm.
Now I know its winter, but she will be in distress so you need to try and keep her as comfy as possible, and as long as you keep her in overnight and keep her warm she will be fine,
Run a bowl/sink full of warm water -then sink her in so her vent is covered,keep her in there for at least 1/2 hour. Mix a baby aspirin and a calcium tablet in water for her. It is really important she doesn't get dehydrated or that will make the problem worse. Keep her in overnight -just fill a cardboard box with straw and keep it somewhere warm and dark where she will not be disturbed.
If she still looks peaky and swollen - and there is no egg, take her to a vet (depending whether you have decided to treat or cull your chickens when ill ) ask around to find one with experience, she will need a look at and treatment will depend on her age/condition. Often a calcuim inj is given plus a course of antibiotics, particulary if she has an infection there.
After make sure you have good quality layers pellets/mash -and this is the main consitute o their diet -watch treats, they dont need them, a few greens to supplement if they do not have access to grass, a little mxed corn in the afternoon is fine in cold weather.
Pullets hatched summer are likely to be coming into lay pretty soon, and, at this time they will be storing calcium ready for their first laying season so it is important to get the diet right at this time. I use old stone flowerpots dotted around filled up with oyster shell so they can peck when they need to.
Finally give her a check over, to ensure there is nothing else unusual - to eliminate an injury.
Good luck!