I have had something like this before and I treated for nutritional - lack of certain vitamins and diet. Some chickens don't get started as well and eventually it affects them. I have seen it happen with chicks and you can have a whole flock that seems fine and one who doesn't. Does she shake her head at all - like she has an ear problem? I was told it can make them feel funny and loopy and that they shake to get rid of the feeling. The walking backwards is a symptom of the same thing and can sometimes be a neurological effect from the vitamin deficiency. Is she eating and drinking okay or just picking at stuff?
When I had this problem with a 2 week old chick about four years ago, I was beside myself. I had an old chicken farmer tell me to buy a vitamin supplement (comes in a little bag and you mix it with water) to put in her water, to isolate her, and to force feed her the vitamins any way I could. I also did a small drop of vitamin E every day or rubed it onto her beak. Within 4 days - she was acting better. It was less than a week. She went to work with me everyday and I fed her six times a day - sometimes forcing it with a dropper. Bread soaked crumbs worked good too. She had another attack of the same thing all of a sudden about a year later and I did the same thing and it worked again. Now Sarafina is a great litle hen - lays well and has not had the isues since then and is 4 years old now.
I can't tell you that this is definitely the problem but it worked for me. Twice!
Hugs to you both!