It could be the surface you are using the mouse on. The way you are holding the mouse. Onset of Parkinson's Disease. A range of different issues which would be the problem.

"What has changed when this started?"

I am guessing surface may be the key issue here for the jerkiness.

I am guessing you restarted your computer and that hasn't cleared it. Does booting up the computer in safemode solve the issue?