My cat has hair missing on her neck?

Hi there here's my story. My cat had kittens dec 9th. i had my daughter 3 weeks later 6 weeks earlier than she was suppose to be. Anyway i could not care for this new baby and 5 kittens so my cat and her kittens went to a foster family through a shelter in the area and once they were about 9 weeks i went to pick up my cat from the shelter. The room they had her in was an open room with probably 40 cats...but she was in a cage and the foster family didnt have any other cats at the house at the time i dont believe. Well when she came home about 2 weeks ago i noticed she was missing a tiny bit of hair on her neck,thought maybe it was from playing with the kittens...well about 5 days ago i noticed it getting bigger(more hair loss) today it looks about 2 inches of hair loss!!! What happened to my baby? Its fleshy looking..im very confused. Going to bring her to the vet tomorrow but just want to see if anyone might have an idea. thanks!