sensitivity is doing wierd things

This is more of a sensitivity bug/error. It has happened to me twice and I didn't even press any thing. I was just trying to get a clownfish when it was getting dark so I quit and turned around and it just happened. its like when you move the mouse it hardly responds and crosshair slides like you don't have control. I tried to change the sensitivity and pressing all the f# buttons and control and alt and fn. I tried every button on my key board but I cant get rid of it by fixing it. I have to wait for it to go back to normal. I am not sure how to recreate it. it just happened.