Since she is 3-4 years old she may have internal laying or egg yolk peritonitis. It's common in hens that age, although I hope that isn't what's going on, but they can walk like an egg bound hen. Here are some links to read:
http://www.chickenvet.co.uk/health-and-common-diseases/egg-laying-issues/index.aspx
http://simplelifeainteasy.blogspot.com/2012/03/egg-yolk-peritonitis-diagnosing-sick.html
https://www.backyardchickens.com/t/526089/egg-yolk-peritonitis
http://www.chickenvet.co.uk/health-and-common-diseases/egg-laying-issues/index.aspx
http://simplelifeainteasy.blogspot.com/2012/03/egg-yolk-peritonitis-diagnosing-sick.html
https://www.backyardchickens.com/t/526089/egg-yolk-peritonitis