My dog has licked the top of her paw until it is an open sore, Why?

It doesn't look as though she was bitten and there is no laceration. She just licks until she has it bleeding. It is an area about the size of a fifty cent piece. It doesn't seem to hurt her, she is walking fine it just looks yucky. Tried putting antibiotic on it but she just licks it off and wrapping it with a bandage is hopeless. Any ideas?