If the bills are in your name then the OR will want to know how much she contributes to the bills so your income shouldn't be questioned. If the bills are joint (i.e. rent is £800) then the OR would expect you both to pay 50%. If this wasn't the case (i.e. your girlfriend says she can only pay 25%) then the OR may ask how much you earn and in extreme cases want to see a recent payslip of yours but this isn't something that normally happens.