in my back \on my back 的区别？

in my back 应该就是指后背，
on sb's back 是指 找某人的麻烦
e.g My wife has been on my back for weeks to buy her a new car.
几星期以来，妻子老是缠着我为她买一件新车。

in my back 一般就是说在背里面,这里把背做为一个身体的部分,比如感觉有疼痛感就说in my back
on my back 一般就是在背上