It may be that the back massager is too powerful. When massaging a back more power is needed to loosen tense muscles and as it does so is supposed to relieve stress. I'm just theorising here, but the power provided in a viberator wouldn't need to be so high because the clitoris is more sensitive. This may be making it sore.
~ The journey of a thousand miles begins with a single step ~
* Nothing in the world has ever been accomplished without passion *