I've just received a letter about benefit fraud?

they say they suspect me of living with someone,there is only my 5yr old child and myself.My childs father and i get on great and as he works away a lot he will come and stay now and then for a couple of days.The thing is he pays a couple of bills by D.D for me,like Sky...more for cartoons really!!...but this was an arrangement made years ago and i dont get the csa after him.To be honest most of his mail comes to here,is this what has caused this?