unwell little pekin chicken

[Crana]

please help our sick little pekin. she is the youngest and smallest of our five hens. Not laying as yet. in the past couple of weeks we keep finding her on her back with her legs in the air! She has had one dose of antibiotics. Wormed. She looks depressed drinking a lot of water. Eating. Isolated her on every occasion and she seems happy to return to the flock after a day. Sleeps a lot. The other chickens appear well. Would appreciate any thoughts. Worried.......