Locked Y axis rotation is also rotating on Z I'm new to blender

How can i get a Z axis only rotation?

I'm trying to make a mug with a handle and when I lock rotation on the y axis I get slight amounts of rotation on Z also; resulting in the handle spiraling as shown below.
From the image it looks like the rotation is normal but your handle is not centered on the mug and that's causing it to look warped.