How long was she at the foster mom's house before you got her? And no problems there? I'd call the foster mom and see what she was feeding. It sounds like it might be environmental if she has gotten progressively worse over the time you've had her. As you said you suspected... maybe it is something in the bedding/carpet/air that is bothering her??
If she were mine, my next step would be to talk to the foster mom and see what she thought!!!
Especially if she had her more than a week or so.