Hi there,
I am sorry to hear of your sick chicken. I got my babies in March of 2009. I had one chicken do the same thing. I did have them all vaccinated for Marek's before they were shipped to me. Now at roughly 4 months of age my rooster Jack had the same issues. He was totally lame and could not stand on his own, he would eat and didn't want to drink. I did lots and lots of research and after feeding him a egg yoke, yogurt, honey, calcium powder and oatmeal diet along with his regular food, and gave him vitamins in his water. I had to force him to drink his water at first but I used a New syringe that I always seem to have on hand for this sort of thing, but with a lot of hard work on both mine and Jack's part, he started walking 2 months after he went lame. I believe it was Rickets which is a calcuim deficiancy and where he was young I was afraid to give my babies anything extra except their food.
Try researching Ricketts and I don't know if it is spelled right, but someone knows something about it. I to was afraid it was Marek's but having noone else had any of the symptons and to this day he is just perfect, because how many roos can you pick up and carry around without them trying to ripe out your eyes, etc....
I hope this may be the problem and it is a fixable problem if it is, but it will take some extra time from your day, but I made sure that my boy made it, and yes I almost gave up on him and then the next week he was sitting on his legs.
gosh I hope the problems is this and not something else.
cause I would love to see your little girl
best of luck to you and your girl and keep a watch on your other ones also.
lisa