My 9 year old son just went through the same thing. We just moved here 6 months ago and his nerves about going to school were very bad. Sometimes he'd be constipated and other times he'd go several times a day complaining of stomach problems. We took him to doctor too and nothing was wrong but stress. Sometimes kids over react or really don't even know what's going on. I tried to make things as comfortable as I could. Talk to her about when you or husband took the bus and fun times on it. Changes are really dramatic to children. It'll take time, but she'll get used to it. Keep telling her how proud you are of her. I talked to my son's teacher, principal, and guidance counselor. Everyone can work together for a child to get through a difficult time. The counselor even told us for him to take 3 deep breaths, close his eyes, sing a part of his favorite song, then tell himself he's safe and he's gonna have a good day.
Good luck and best wishes!!