I'm so sorry she is not doing well
The stance in the photo of her standing up and with her being an Ex-Batt, I would take a guess that she is suffering from a reproductive disorder. Internal laying, Egg Yolk Peritonitis, cancer, tumors and Salpingitis are common in laying hens.
Do you note any swelling/bloat or feeling of fluid in her abdomen?
If you can have a fecal float to check for worms that would be good as well.
Unfortunately, if she is suffering from repro problems, all you can do is offer supportive care. If she is having difficulty breathing, it may be time to let her go.
Sending the body for necropsy will give you some answers as to what was going on with her.
Are you located in the US?
http://www.chickenvet.co.uk/health-and-common-diseases/egg-laying-issues/index.aspx