If she seems terrified of the rooster, can you separate him for a few days? An enlarged lower belly between the legs can be fluid or internal laying, or just fat. Having a lot of white urates in the poop can sometimes be a sign of dehydration.
I encourage drinking or eating by holding a small cup or lid up to the beak. You can dip their beak for a second to get the message. A tuna or cat food can can be used for this. Some hold a spoon up to the side of the beak. Tube feeding is something that can be easily learned.
You don’t have to vaccinate for Mareks. Many do when they come from a hatchery, but many do not, especially if the chicks come from a feed store or a breeder, or are hatched at home. Hopefully, it is an injury. Since she may be injured, she may do better in a dog crate where you can prevent her...
Have you added any new birds to the flock in the last few months? Have you ever had an unexpected death or a lame chicken before? Was she vaccinated for Mareks disease? Make sure that she can get to food and water. Let us know if she bets worse.