My rescued rabbit has a constant poo bum?

I have to wash her nearly every day to keep her clean, was wondering if anyone had any advise before she sees the vet again please. she is slightly overweight as she had been stuck in a hutch on her own for months before i went and got her. She has free running space 24/7 but didnt know what she ate before i got her. can i settle her stomach in any way?