Flips on back

[mwendorffr]

A few days ago I found one of my year old chickens on her back. I put her back on her feet and she tried to run away and flipped on her back again. I put her on her feet again and she went outside and was fine the rest of the day. Today when I went to lock my flock up for the night I found her hidden in a corner. She looks lethargic, can't walk, when she tries to run away she flips on her back. She has a dirty butt covered in loose crap. She's not eating or drinking. I couldn't find any mites or fleas on her and not egg bound.
I had this happen 3 weeks ago to my rooster which I culled. I'm not sure what's wrong. I'm getting some chicks on Monday and worried this is something that will spread to my whole flock and these chicks when they are ready to be integrated.
Does anyone have an idea what's wrong?