What's wrong with my chicken?

one of my chickens started acting strangely a couple of days ago, firstly it was almost as if she was drunk and I thought she may have eaten some fermented berries or something (they're free range) so I shut her in the hutch. The next day there was no change but I was working so couldn't monitor her properly. Today however she's taken a turn for the worse and could barely make it out of the hutch it was like her legs weren't working properly. I've separated her from the others, who are all acting normally and got her in a cardboard box in the house, I've checked her over and she seems fine apart from her keel which is bare and looks red and quite sore. She is eating fine but she can't sit properly and sits with her legs to the side and looks very uncomfortable. She's an Andalusian bantam who normally hates to be picked up but now she doesn't even struggle when I pick her up. If anyone knows anything else to look for please let me know. If you know what it is and how to treat it that would be great! Thanks.