Chicken unwell very swollen between legs and waddling

I have a 4 year old hybrid brown chicken who 2 days ago started waddling like a duck. To cut a long story short, over the last 2 days her back end has dropped lower and lower until she is now more or less dragging her undercarriage along the ground! She is very very swollen between her legs and is very wet and mucky down her backend all the time. I've looked where she has been sleeping at night and noticed she she is only passing white liquid poo despite the fact that she can be tempted to eat with tasty bits and pieces and is still drinking ok.

I've never had an eggbound chicken and thought it might be this, however after a warm bath 2 days running, nothing has transpired and she seems to be dragging herself more and more. Strangely she doesn't seem too miserable although she's much happier to be handled than she normally is. Vets are all shut for bank holiday now and I'm wondering if anyone has any advice??? Please...

Thanks for your reply. Looking up chicken peritonitis on google and extensively on here, it definitely looks like this is what she has.

Does anyone have any further information about this:

Will antibiotics cure the peritonitis?

Will she still lay or is it more humane to have her put to sleep at this stage?

Poor Martha! :-(