My cat scratches and digs at herself?

she has been doing this for about 2 months, she has no hair on her belly or backside. she also has scabs where she has been digging. she has no fleas or anything else i can see on her to make her itch so bad, i also have to other cats and they are fine, I am not sure what her problem could be so maybe someone else does. please help she is my baby.