Help! Sick 5 year old chicken.

Hi Everyone-

I have a 5 year old chicken that has seemed very healthy until this am. She looked like she was trying to lay an egg but wasn't
in a nest box. I picked her up and she is very thin. She also didn't try to get away which is unusual for her. I checked for signs that she was egg bound but, didn't feel an egg. Her tail was straight down and she seemed to be leaning backwards. I put her by her water and she drank a little. She has food in her crop but, didn't eat when I put her by her food. I separated her from the roosters so she didn't get pushed around and left. When I got home tonight she was sitting on her butt with her legs out in front of her. She can stand and walk but is very weak. She would not eat her food but when I cracked open an egg she ate some of it. I looked her over for injury and didn't see anything. I did find two small mite like bugs on her. I looked into some info on mites when I found them but, she is not picking out her feathers, her coloring is good in her comb and I only found the two. I don't know what else to look for and have no idea what is going on. If anyone has any ideas please help. She is the only chicken I have left from my first batch of chickens and very special to me.

Thanks- Jessica