What type of skin condition can cause a dog to lose hair and itch badly?

we have a lab/pitt mix that is an indoor dog and has developed a skin condition on her back that goes all the way down to her tail. we have tried antibiotic ointment, anti itch doggie shampoo and medicated shampoo from the vet and it still seems to drive her crazy. She chews until she pulls out hair and you can tell that that area is painful. Does anyone have any suggestions to what this may be or what I can do for her?