Hi, sorry your having trouble. I actually just put down a chicken within the hour because she was lethargic, and was "waddling" & When we picked her up we found her lower abdomen was severely swollen. We tried the methods for helping pass a bound egg but nothing helped and she deteriorated quickly. So we made the hard decision to let her go. However, we decided to investigate the cause and found she had clear fluid in her abdomen that had filled her body cavity. Often Called water belly.
Check your chicken for swollen belly and be sure that she does not have an egg bound. There are steps you can take to help her pass an egg such as a warm bath and massage or using Vaseline, oil or lubricant and putting it inside her vent. You can find videos on how to do this on you tube.
We didn't not have a choice but to put ours down because we did not catch the cause soon enough. But it sounds like your may be in early stages.
Good luck !!
I would just keep an eye on her. Could be arthritis as mentioned or she may have something else going on internally. Does she feel thin at all, sharp breast bone? Has she been dewormed and checked for mites?
As far as peritonitis, in general there isn't much you can do other then try to make them more comfortable. There is always an underlying cause and those things are rarely fixable. However, if her lower abdomen, from between the legs back to the vent area, feels normal, no swelling, no redness, then I wouldn't jump to that conclusion.