in my back \on my back 的区别?

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