From my experience, your image is indicating a collimation error. If your polar alignment is accurate, rotation is out. Flexure is out, as the error is not across the field (assuming you are using a guide scope with the F4). A tilt in the ccd will translate to an effect across the field (according to CCDInspector).
I have had this exact issue but with a different scope, and many claimed it to be rotation, but with excellent polar alignment, that's not likely. If the star elongation doesn't extend with longer exposures, it's not rotation.
What I found to cause this in my case was the optical axis failed to centre to the secondary mirror. Attempting to collimate under this circumstance caused this unusual elongation to one side of the field. Yours is a Newt, so I'm not sure the same can be said (mine is a CDK).
Good luck with it.
Steve.
