Everyone else has given great advise. My two cents will be, have her checked by a vet for a UTI. Sometimes housebroken dogs will potty in the house because they feel the urge to go and because it's their way of letting you know something is wrong. Unlike cats who try to go and nothing comes out, when a dog has a UTI, they go. Or if she isn't spayed, she could be marking her territory. Sounds weird, but some females do it in which case spaying will help.