Hi I had so much luck with my last sick girl I came back for a new issue!

My 8-9 month old Buff Orpington woke this morning not being able to walk without falling over. She is standing but cannot walk without falling. She was totally fine yesterday! Her comb is bright red.. drinking but not eating. She seems alert and normal, pupils are very dilated but maybe because she is in the house. She is eating a bit too if I put food up high so she doesn’t have to bend. She seems totally fine outside of the fact that she cannot walk. Could she be egg bound? Everyone in flock is fine... only changes.. been cold and snowy.. I added a hay bale to run yesterday... from feed store. Could it be poison? That may sound silly.. just so worried. Any help would be appreciated please! So worried about my Gertrude!